[#2] Shallow clones are compatible with lockfiles

GitHub must have added the ability to fetch an unadvertised ref at
some point, because it sure seems to work now when it didn't before.
We take advantage in straight.el.

README.md

@@ -1866,9 +1866,9 @@ These are the keywords meaningful for the `git` backend:
   `:branch`, and `:remote`.
 * `:depth`: either the symbol `full` or an integer. If `full`, then
   the repository is cloned with its whole history. If an integer `N`,
-  then the repository is cloned with the option `--depth N`, unless a
-  commit is specified (e.g. by version lockfiles). The default value
-  is `full`.
+  then the repository is cloned with the option `--depth N`. This
+  works even when a commit is specified (e.g. by version lockfiles).
+  The default value is `full`.
 
 This section tells you how the `git` backend, specifically, implements
 the version-control backend API:
@@ -1937,8 +1937,8 @@ You can customize the following user options:
 * `straight-vc-git-default-clone-depth`: the default value for the
   `:depth` keyword. It can be either the symbol `full` or an integer,
   and defaults to `full`. Setting this variable to a small integer will
-  reduce the size of repositories. Note that this variable does *not*
-  affect packages whose versions are locked.
+  reduce the size of repositories. This variable affects all packages,
+  even those whose versions are locked.
 
 ##### Deprecated `:upstream` keyword
 

straight.el

@@ -2139,36 +2139,54 @@ unless a commit is specified (e.g. by version lockfiles)."
   :type '(choice integer (const full)))
 
 (cl-defun straight-vc-git--clone-internal
-    (&key depth upstream-remote url repo-dir branch)
+    (&key depth remote url repo-dir branch commit)
   "Clone a remote repository from URL.
 
-If DEPTH is the symbol `full', clone the whole history of the repository.
-If DEPTH is an integer, clone with the option --depth DEPTH --branch BRANCH.
-If this fails, try again to clone without the option --depth and --branch,
-as a fallback.
+If DEPTH is the symbol `full', clone the whole history of the
+repository. If DEPTH is an integer, pass it to the --depth option
+of git-clone to perform a shallow clone. If this fails, try again
+to clone without the option --depth and --branch, as a fallback.
 
-UPSTREAM-REMOTE is the name of the remote to use for the upstream
-\(e.g. \"origin\"; see `straight-vc-git-default-remote-name').
-URL and REPO-DIR are the positional arguments passed to
-git-clone(1), and BRANCH is the name of the default
-branch (although it won't be checked out as per --no-checkout)."
+REMOTE is the name of the remote to use \(e.g. \"origin\"; see
+`straight-vc-git-default-remote-name'). URL and REPO-DIR are the
+positional arguments passed to git-clone(1), and BRANCH is the
+name of the default branch (although it won't be checked out as
+per --no-checkout).
+
+If COMMIT is non-nil and DEPTH is not `full', then try to clone
+only that specific commit from the remote. Fall back to doing a
+clone of everything."
   (cond
    ((eq depth 'full)
     ;; Clone the whole history of the repository.
     (straight--get-call
-     "git" "clone" "--origin" upstream-remote
+     "git" "clone" "--origin" remote
      "--no-checkout" url repo-dir))
    ((integerp depth)
     ;; Do a shallow clone.
     (condition-case nil
-        (straight--get-call
-         "git" "clone" "--origin" upstream-remote
-         "--no-checkout" url repo-dir
-         "--depth" (number-to-string depth)
-         "--branch" branch)
+        (if commit
+            (progn
+              (make-directory repo-dir)
+              (let ((straight--default-directory nil)
+                    (default-directory repo-dir))
+                (straight--get-call
+                 "git" "init")
+                (straight--get-call
+                 "git" "remote" "add" remote url
+                 "--master" branch)
+                (straight--get-call
+                 "git" "fetch" remote commit
+                 "--depth" (number-to-string depth))))
+          (delete-directory repo-dir 'recursive)
+          (straight--get-call
+           "git" "clone" "--origin" remote
+           "--no-checkout" url repo-dir
+           "--depth" (number-to-string depth)
+           "--branch" branch))
       ;; Fallback for dumb http protocol.
       (error (straight-vc-git--clone-internal :depth 'full
-                                              :upstream-remote upstream-remote
+                                              :remote remote
                                               :url url
                                               :repo-dir repo-dir))))
    (t (error "Invalid value %S of depth for %s" depth url))))
@@ -2182,29 +2200,28 @@ out, signal a warning. If COMMIT is nil, check out the branch
 specified in RECIPE instead. If that fails, signal a warning."
   (straight-vc-git--destructure recipe
       (package local-repo branch remote upstream-repo upstream-host
-               upstream-remote fork-repo fork-host
-               fork-remote nonrecursive depth)
+               upstream-remote fork-repo repo host nonrecursive depth)
     (unless upstream-repo
       (error "No `:repo' specified for package `%s'" package))
     (let ((success nil)
           (repo-dir (straight--repos-dir local-repo))
-          (url (straight-vc-git--encode-url upstream-repo upstream-host))
-          (depth (or (when commit 'full)
-                     depth
-                     straight-vc-git-default-clone-depth)))
+          (url (straight-vc-git--encode-url repo host))
+          (depth (or depth straight-vc-git-default-clone-depth)))
       (unwind-protect
           (progn
             (straight-vc-git--clone-internal :depth depth
-                                             :upstream-remote upstream-remote
+                                             :remote remote
                                              :url url
                                              :repo-dir repo-dir
-                                             :branch branch)
+                                             :branch branch
+                                             :commit commit)
             (let ((straight--default-directory nil)
                   (default-directory repo-dir))
               (when fork-repo
-                (let ((url (straight-vc-git--encode-url fork-repo fork-host)))
-                  (straight--get-call "git" "remote" "add" fork-remote url)
-                  (straight--get-call "git" "fetch" fork-remote)))
+                (let ((url (straight-vc-git--encode-url
+                            upstream-repo upstream-host)))
+                  (straight--get-call "git" "remote" "add" upstream-remote url)
+                  (straight--get-call "git" "fetch" upstream-remote)))
               (when commit
                 (unless (straight--check-call "git" "checkout" commit)
                   (straight--warn