parse-options: allow --end-of-options as a synonym for "--"

The revision option parser recently learned about --end-of-options, but
that's not quite enough for all callers. Some of them, like git-log,
pick out some options using parse_options(), and then feed the remainder
to setup_revisions(). For those cases we need to stop parse_options()
from finding more options when it sees --end-of-options, and to retain
that option in argv so that setup_revisions() can see it as well.

Let's handle this the same as we do "--". We can even piggy-back on the
handling of PARSE_OPT_KEEP_DASHDASH, because any caller that wants to
retain one will want to retain the other.

I've included two tests here. The "log" test covers "--source", which is
one of the options it handles with parse_options(), and would fail
before this patch. There's also a test that uses the parse-options
helper directly. That confirms that the option is handled correctly even
in cases without KEEP_DASHDASH or setup_revisions(). I.e., it is safe to
use --end-of-options in place of "--" in other programs.

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

parse-options.c

@@ -780,7 +780,8 @@ int parse_options_step(struct parse_opt_ctx_t *ctx,
 			continue;
 		}
 
-		if (!arg[2]) { /* "--" */
+		if (!arg[2] /* "--" */ ||
+		    !strcmp(arg + 2, "end-of-options")) {
 			if (!(ctx->flags & PARSE_OPT_KEEP_DASHDASH)) {
 				ctx->argc--;
 				ctx->argv++;