Use the "real" git of Git for Windows 2+

Git for Windows 2.x reorganized the directory structure so both cmd/git
and bin/git are just wrappers that call the real thing.  Try to find and
use the "real" one instead (it's faster) by executing `which' via the
wrapper.  We also need to set PATH so that git can find all its
utitilities and dlls, this can be retrieved from the wrapper as well.

close #2259

lisp/magit-git.el

@@ -50,6 +50,9 @@
   "Git and other external processes used by Magit."
   :group 'magit)
 
+(defvar magit-git-environment nil
+  "Prepended to `process-environment' while running git.")
+
 (defcustom magit-git-executable
   ;; Git might be installed in a different location on a remote, so
   ;; it is better not to use the full path to the executable, except
@@ -58,14 +61,28 @@
   ;; than using "bin/git.exe" directly.
   (or (and (eq system-type 'windows-nt)
            (--when-let (executable-find "git.exe")
-             (let ((alt (directory-file-name (file-name-directory it))))
-               (if (and (equal (file-name-nondirectory alt) "cmd")
-                        (setq alt (expand-file-name
-                                   (convert-standard-filename "bin/git.exe")
-                                   (file-name-directory alt)))
-                        (file-executable-p alt))
-                   alt
-                 it))))
+             (or (with-temp-buffer
+                   (when (save-excursion
+                           (= (call-process
+                              it nil '(t t) nil "-c"
+                              "alias.exe=!which git | cygpath -wf -" "exe") 0))
+                     (prog1 (delete-and-extract-region 1 (line-end-position))
+                       (save-excursion
+                         (insert "PATH=")
+                         (call-process
+                          it nil '(t t) nil "-c"
+                          "alias.path=!cygpath -wp \"$PATH\"" "path"))
+                       (setq magit-git-environment
+                             (buffer-substring-no-properties
+                              (point) (line-end-position))))))
+                 (let ((alt (directory-file-name (file-name-directory it))))
+                   (if (and (equal (file-name-nondirectory alt) "cmd")
+                            (setq alt (expand-file-name
+                                       (convert-standard-filename "bin/git.exe")
+                                       (file-name-directory alt)))
+                            (file-executable-p alt))
+                       alt
+                     it)))))
       "git")
   "The Git executable used by Magit."
   :group 'magit-process

lisp/magit-process.el

@@ -265,12 +265,13 @@ Identical to `process-file' but temporarily enable Cygwin's
     (apply #'process-file args)))
 
 (defun magit-cygwin-env-vars ()
-  (when magit-need-cygwin-noglob
-    (mapcar (lambda (var)
-              (concat var "=" (--if-let (getenv var)
-                                  (concat it " noglob")
-                                "noglob")))
-            '("CYGWIN" "MSYS"))))
+  (append magit-git-environment
+          (when magit-need-cygwin-noglob
+            (mapcar (lambda (var)
+                      (concat var "=" (--if-let (getenv var)
+                                          (concat it " noglob")
+                                        "noglob")))
+                    '("CYGWIN" "MSYS")))))
 
 (defun magit-run-git-with-input (input &rest args)
   "Call Git in a separate process.