pretty: treat "--format=" as an empty userformat

Until now, we treated "--pretty=" or "--format=" as "give me
the default format". This was not planned nor documented,
but only what happened to work due to our parsing of
"--pretty" (which should give the default format).

Let's instead let these be an actual empty userformat.
Otherwise one must write out the annoyingly long
"--pretty=tformat:" to get the same behavior.

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

pretty.c

@@ -146,7 +146,7 @@ void get_commit_format(const char *arg, struct rev_info *rev)
 	struct cmt_fmt_map *commit_format;
 
 	rev->use_terminator = 0;
-	if (!arg || !*arg) {
+	if (!arg) {
 		rev->commit_format = CMIT_FMT_DEFAULT;
 		return;
 	}
@@ -155,7 +155,7 @@ void get_commit_format(const char *arg, struct rev_info *rev)
 		return;
 	}
 
-	if (strchr(arg, '%')) {
+	if (!*arg || strchr(arg, '%')) {
 		save_user_format(rev, arg, 1);
 		return;
 	}