add git_path_buf helper function

If you have a function that uses git_path a lot, but would
prefer to avoid the static buffers, it's useful to keep a
single scratch buffer locally and reuse it for each call.
You used to be able to do this with git_snpath:

  char buf[PATH_MAX];

  foo(git_snpath(buf, sizeof(buf), "foo"));
  bar(git_snpath(buf, sizeof(buf), "bar"));

but since 1a83c24, git_snpath has been replaced with
strbuf_git_path. This is good, because it removes the
arbitrary PATH_MAX limit. But using strbuf_git_path is more
awkward for two reasons:

  1. It adds to the buffer, rather than replacing it. This
     is consistent with other strbuf functions, but makes
     reuse of a single buffer more tedious.

  2. It doesn't return the buffer, so you can't format
     as part of a function's arguments.

The new git_path_buf solves both of these, so you can use it
like:

  struct strbuf buf = STRBUF_INIT;

  foo(git_path_buf(&buf, "foo"));
  bar(git_path_buf(&buf, "bar"));

  strbuf_release(&buf);

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -723,6 +723,8 @@ extern char *mksnpath(char *buf, size_t n, const char *fmt, ...)
 	__attribute__((format (printf, 3, 4)));
 extern void strbuf_git_path(struct strbuf *sb, const char *fmt, ...)
 	__attribute__((format (printf, 2, 3)));
+extern char *git_path_buf(struct strbuf *buf, const char *fmt, ...)
+	__attribute__((format (printf, 2, 3)));
 extern void strbuf_git_path_submodule(struct strbuf *sb, const char *path,
 				      const char *fmt, ...)
 	__attribute__((format (printf, 3, 4)));

path.c

@@ -175,6 +175,16 @@ static void do_git_path(struct strbuf *buf, const char *fmt, va_list args)
 	strbuf_cleanup_path(buf);
 }
 
+char *git_path_buf(struct strbuf *buf, const char *fmt, ...)
+{
+	va_list args;
+	strbuf_reset(buf);
+	va_start(args, fmt);
+	do_git_path(buf, fmt, args);
+	va_end(args);
+	return buf->buf;
+}
+
 void strbuf_git_path(struct strbuf *sb, const char *fmt, ...)
 {
 	va_list args;