Strbuf API extensions and fixes.

* Add strbuf_rtrim to remove trailing spaces.
  * Add strbuf_insert to insert data at a given position.
  * Off-by one fix in strbuf_addf: strbuf_avail() does not counts the final
    \0 so the overflow test for snprintf is the strict comparison. This is
    not critical as the growth mechanism chosen will always allocate _more_
    memory than asked, so the second test will not fail. It's some kind of
    miracle though.
  * Add size extension hints for strbuf_init and strbuf_read. If 0, default
    applies, else:
      + initial buffer has the given size for strbuf_init.
      + first growth checks it has at least this size rather than the
        default 8192.

Signed-off-by: Pierre Habouzit <madcoder@debian.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-checkout-index.c

@@ -274,7 +274,7 @@ int cmd_checkout_index(int argc, const char **argv, const char *prefix)
 		struct strbuf buf;
 		if (all)
 			die("git-checkout-index: don't mix '--all' and '--stdin'");
-		strbuf_init(&buf);
+		strbuf_init(&buf, 0);
 		while (1) {
 			char *path_name;
 			const char *p;