repository: add function to iterate over all HEADs

While we already provide functions to get the current repository's HEAD,
it is quite involved to iterate over HEADs of both the repository and
all linked work trees. This commit implements a function
`git_repository_foreach_head`, which accepts a callback which is then
called for all HEAD files.

src/repository.c

@@ -2137,6 +2137,38 @@ out:
 	return error;
 }
 
+int git_repository_foreach_head(git_repository *repo, git_repository_foreach_head_cb cb, void *payload)
+{
+	git_strarray worktrees = GIT_VECTOR_INIT;
+	git_buf path = GIT_BUF_INIT;
+	int error;
+	size_t i;
+
+	/* Execute callback for HEAD of commondir */
+	if ((error = git_buf_joinpath(&path, repo->commondir, GIT_HEAD_FILE)) < 0 ||
+	    (error = cb(repo, path.ptr, payload) != 0))
+		goto out;
+
+	if ((error = git_worktree_list(&worktrees, repo)) < 0) {
+		error = 0;
+		goto out;
+	}
+
+	/* Execute callback for all worktree HEADs */
+	for (i = 0; i < worktrees.count; i++) {
+		if (get_worktree_file_path(&path, repo, worktrees.strings[i], GIT_HEAD_FILE) < 0)
+			continue;
+
+		if ((error = cb(repo, path.ptr, payload)) != 0)
+			goto out;
+	}
+
+out:
+	git_buf_free(&path);
+	git_strarray_free(&worktrees);
+	return error;
+}
+
 int git_repository_head_unborn(git_repository *repo)
 {
 	git_reference *ref = NULL;

src/repository.h

@@ -159,6 +159,26 @@ GIT_INLINE(git_attr_cache *) git_repository_attr_cache(git_repository *repo)
 int git_repository_head_tree(git_tree **tree, git_repository *repo);
 int git_repository_create_head(const char *git_dir, const char *ref_name);
 
+/*
+ * Called for each HEAD.
+ *
+ * Can return either 0, causing the iteration over HEADs to
+ * continue, or a non-0 value causing the iteration to abort. The
+ * return value is passed back to the caller of
+ * `git_repository_foreach_head`
+ */
+typedef int (*git_repository_foreach_head_cb)(git_repository *repo, const char *path, void *payload);
+
+/*
+ * Iterate over repository and all worktree HEADs.
+ *
+ * This function will be called for the repository HEAD and for
+ * all HEADS of linked worktrees. For each HEAD, the callback is
+ * executed with the given payload. The return value equals the
+ * return value of the last executed callback function.
+ */
+int git_repository_foreach_head(git_repository *repo, git_repository_foreach_head_cb cb, void *payload);
+
 /*
  * Weak pointers to repository internals.
  *