fetchgitlocal: fix permission problem

pkgs/build-support/fetchgitlocal/default.nix

@@ -1,27 +1,26 @@
 { runCommand, git, nix }: src:
 
 let
+  tmpFolder =  "/tmp/fetchgitlocal-${builtins.toString currentTime}";
+  currentTime = builtins.currentTime; # impure, do every time
+
   srcStr = toString src;
 
-  # Adds the current directory (respecting ignored files) to the git store, and returns the hash
+  # Adds the current directory in the index (respecting ignored files) to the git store,
+  # and returns the hash
   gitHashFile = runCommand "put-in-git" {
       nativeBuildInputs = [ git ];
-      dummy = builtins.currentTime; # impure, do every time
+      dummy = currentTime;
       preferLocalBuild = true;
     } ''
-      cd ${srcStr}
-      DOT_GIT=$(git rev-parse --resolve-git-dir .git) # path to repo
-
-      cp $DOT_GIT/index $DOT_GIT/index-user # backup index
-      git reset # reset index
-      git add . # add current directory
-
-      # hash of current directory
-      # remove trailing newline
-      git rev-parse $(git write-tree) \
-        | tr -d '\n' > $out
-
-      mv $DOT_GIT/index-user $DOT_GIT/index # restore index
+      # we need write access to update index
+      cp -r ${srcStr}/.git ${tmpFolder}
+      chmod a+w ${tmpFolder}
+      export GIT_DIR=${tmpFolder}
+
+      # `tr` to remove trailing newline
+      res="$(git rev-parse --show-prefix)"
+      git write-tree --prefix="$res" | tr -d '\n' > $out
     '';
 
   gitHash = builtins.readFile gitHashFile; # cache against git hash
@@ -32,9 +31,13 @@ let
     } ''
       mkdir $out
 
+      # git annoyingly breaks without doing this since the hash does
+      # not correspond to repo root.
+      cd $(git -C ${srcStr} rev-parse --show-toplevel)
+
       # dump tar of *current directory* at given revision
-      git -C ${srcStr} archive --format=tar ${gitHash} \
-        | tar xf - -C $out
+      hash="${gitHash}"
+      git archive --format=tar $hash | tar xv -C $out
     '';
 
 in nixPath