worktree: return allocated string from get_worktree_git_dir()

The `get_worktree_git_dir()` function returns a string constant that
does not need to be free'd by the caller. This string is computed for
three different cases:

  - If we don't have a worktree we return a path into the Git directory.
    The returned string is owned by `the_repository`, so there is no
    need for the caller to free it.

  - If we have a worktree, but no worktree ID then the caller requests
    the main worktree. In this case we return a path into the common
    directory, which again is owned by `the_repository` and thus does
    not need to be free'd.

  - In the third case, where we have an actual worktree, we compute the
    path relative to "$GIT_COMMON_DIR/worktrees/". This string does not
    need to be released either, even though `git_common_path()` ends up
    allocating memory. But this doesn't result in a memory leak either
    because we write into a buffer returned by `get_pathname()`, which
    returns one out of four static buffers.

We're about to drop `git_common_path()` in favor of `repo_common_path()`,
which doesn't use the same mechanism but instead returns an allocated
string owned by the caller. While we could adapt `get_worktree_git_dir()`
to also use `get_pathname()` and print the derived common path into that
buffer, the whole schema feels a lot like premature optimization in this
context. There are some callsites where we call `get_worktree_git_dir()`
in a loop that iterates through all worktrees. But none of these loops
seem to be even remotely in the hot path, so saving a single allocation
there does not feel worth it.

Refactor the function to instead consistently return an allocated path
so that we can start using `repo_common_path()` in a subsequent commit.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/receive-pack.c

@@ -1435,7 +1435,8 @@ static const char *push_to_checkout(unsigned char *hash,
 
 static const char *update_worktree(unsigned char *sha1, const struct worktree *worktree)
 {
-	const char *retval, *git_dir;
+	const char *retval;
+	char *git_dir;
 	struct strvec env = STRVEC_INIT;
 	int invoked_hook;
 
@@ -1453,6 +1454,7 @@ static const char *update_worktree(unsigned char *sha1, const struct worktree *w
 		retval = push_to_deploy(sha1, &env, worktree->path);
 
 	strvec_clear(&env);
+	free(git_dir);
 	return retval;
 }