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>