worktree: error out early if given ref is not valid

When adding a new worktree, we only verify that an optionally given
reference is valid half-way through the function. At this point, some
data structures have already been created on-disk. If we bail out due to
an invalid reference, these will be left behind and need to be manually
cleaned up by the user.

Improve the situation by moving the reference checks to the function's
preamble. Like this, we error out as early as possible and will not
leave behind any files.

src/worktree.c

@@ -290,6 +290,20 @@ int git_worktree_add(git_worktree **out, git_repository *repo,
 
 	*out = NULL;
 
+	if (wtopts.ref) {
+		if (!git_reference_is_branch(wtopts.ref)) {
+			git_error_set(GIT_ERROR_WORKTREE, "reference is not a branch");
+			err = -1;
+			goto out;
+		}
+
+		if (git_branch_is_checked_out(wtopts.ref)) {
+			git_error_set(GIT_ERROR_WORKTREE, "reference is already checked out");
+			err = -1;
+			goto out;
+		}
+	}
+
 	/* Create gitdir directory ".git/worktrees/<name>" */
 	if ((err = git_buf_joinpath(&gitdir, repo->commondir, "worktrees")) < 0)
 		goto out;
@@ -342,18 +356,6 @@ int git_worktree_add(git_worktree **out, git_repository *repo,
 
 	/* Set up worktree reference */
 	if (wtopts.ref) {
-		if (!git_reference_is_branch(wtopts.ref)) {
-			git_error_set(GIT_ERROR_WORKTREE, "reference is not a branch");
-			err = -1;
-			goto out;
-		}
-
-		if (git_branch_is_checked_out(wtopts.ref)) {
-			git_error_set(GIT_ERROR_WORKTREE, "reference is already checked out");
-			err = -1;
-			goto out;
-		}
-
 		if ((err = git_reference_dup(&ref, wtopts.ref)) < 0)
 			goto out;
 	} else {