path: add repo_worktree_path and strbuf_repo_worktree_path

Introduce 'repo_worktree_path' and 'strbuf_repo_worktree_path' which
take a repository struct and constructs a path relative to the
repository's worktree.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

path.c

@@ -506,6 +506,47 @@ const char *worktree_git_path(const struct worktree *wt, const char *fmt, ...)
 	return pathname->buf;
 }
 
+static void do_worktree_path(const struct repository *repo,
+			     struct strbuf *buf,
+			     const char *fmt, va_list args)
+{
+	strbuf_addstr(buf, repo->worktree);
+	if(buf->len && !is_dir_sep(buf->buf[buf->len - 1]))
+		strbuf_addch(buf, '/');
+
+	strbuf_vaddf(buf, fmt, args);
+	strbuf_cleanup_path(buf);
+}
+
+char *repo_worktree_path(const struct repository *repo, const char *fmt, ...)
+{
+	struct strbuf path = STRBUF_INIT;
+	va_list args;
+
+	if (!repo->worktree)
+		return NULL;
+
+	va_start(args, fmt);
+	do_worktree_path(repo, &path, fmt, args);
+	va_end(args);
+
+	return strbuf_detach(&path, NULL);
+}
+
+void strbuf_repo_worktree_path(struct strbuf *sb,
+			       const struct repository *repo,
+			       const char *fmt, ...)
+{
+	va_list args;
+
+	if (!repo->worktree)
+		return;
+
+	va_start(args, fmt);
+	do_worktree_path(repo, sb, fmt, args);
+	va_end(args);
+}
+
 /* Returns 0 on success, negative on failure. */
 static int do_submodule_path(struct strbuf *buf, const char *path,
 			     const char *fmt, va_list args)

path.h

@@ -43,6 +43,14 @@ extern void strbuf_repo_git_path(struct strbuf *sb,
 				 const char *fmt, ...)
 	__attribute__((format (printf, 3, 4)));
 
+extern char *repo_worktree_path(const struct repository *repo,
+				const char *fmt, ...)
+	__attribute__((format (printf, 2, 3)));
+extern void strbuf_repo_worktree_path(struct strbuf *sb,
+				      const struct repository *repo,
+				      const char *fmt, ...)
+	__attribute__((format (printf, 3, 4)));
+
 extern void report_linked_checkout_garbage(void);
 
 /*