builtins.fetchGit: Fix build when fetching a git worktree

Worktrees[1] are a feature of git which allow you to check out a ref in
a different directory.

While playing around with flakes I realized that git repositories in a
worktree checkout break when trying to build a flake:

```
$ git worktree add ../nixpkgs-flakes nixpkgs-flakes
$ cd ../nixpkgs-flakes
$ nix build .#hello
error: opening directory '/home/ma27/Projects/nixpkgs-flakes/.git/refs/heads': Not a directory
```

This issue has been fixed by determining with `git rev-parse --git-common-dir`
where the actual `.git` directory is.

Please note that this issue only exists on the `flakes` branch, fetching
worktree checkouts with Nix master seems to work fine.

[1] https://git-scm.com/docs/git-worktree

src/libexpr/primops/fetchGit.cc

@@ -92,7 +92,22 @@ GitInfo exportGit(ref<Store> store, std::string uri,
 
     if (hasPrefix(uri, "git+")) uri = std::string(uri, 4);
 
-    bool isLocal = hasPrefix(uri, "/") && pathExists(uri + "/.git");
+    bool isLocalPath = hasPrefix(uri, "/");
+    auto gitDir = uri + "/.git";
+    if (isLocalPath) {
+        auto commonDir = chomp(runProgram(
+            "git",
+            true,
+            { "-C", uri, "rev-parse", "--git-common-dir" }
+        ));
+
+        // if URI is not in a worktree, `git rev-parse --git-common-dir` always
+        // returns `.git`, even if `-C` is set :(
+        if (commonDir != ".git")
+            gitDir = commonDir;
+    }
+
+    bool isLocal = isLocalPath && pathExists(gitDir);
 
     // If this is a local directory (but not a file:// URI) and no ref
     // or revision is given, then allow the use of an unclean working
@@ -102,7 +117,7 @@ GitInfo exportGit(ref<Store> store, std::string uri,
 
         /* Check whether this repo has any commits. There are
            probably better ways to do this. */
-        bool haveCommits = !readDirectory(uri + "/.git/refs/heads").empty();
+        bool haveCommits = !readDirectory(gitDir + "/refs/heads").empty();
 
         try {
             if (haveCommits) {

tests/fetchGit.sh

@@ -25,8 +25,14 @@ rev1=$(git -C $repo rev-parse HEAD)
 
 echo world > $repo/hello
 git -C $repo commit -m 'Bla2' -a
+git -C $repo worktree add $TEST_ROOT/worktree
+echo hello >> $TEST_ROOT/worktree/hello
 rev2=$(git -C $repo rev-parse HEAD)
 
+# Fetch a dirty worktree
+path0=$(nix eval --impure --raw "(builtins.fetchGit $TEST_ROOT/worktree).outPath")
+[[ $(tail -n 1 $path0/hello) = "hello" ]]
+
 # Fetch the default branch.
 path=$(nix eval --impure --raw "(builtins.fetchGit file://$repo).outPath")
 [[ $(cat $path/hello) = world ]]