log --pretty: do not accept bogus "--prettyshort"

... nor bogus "format.pretty = '=short'".  Both are syntax errors.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

pretty.c

@@ -28,8 +28,6 @@ void get_commit_format(const char *arg, struct rev_info *rev)
 		rev->commit_format = CMIT_FMT_DEFAULT;
 		return;
 	}
-	if (*arg == '=')
-		arg++;
 	if (!prefixcmp(arg, "format:") || !prefixcmp(arg, "tformat:")) {
 		const char *cp = strchr(arg, ':') + 1;
 		free(user_format);

revision.c

@@ -1197,11 +1197,16 @@ int setup_revisions(int argc, const char **argv, struct rev_info *revs, const ch
 				revs->verbose_header = 1;
 				continue;
 			}
-			if (!prefixcmp(arg, "--pretty")) {
+			if (!strcmp(arg, "--pretty")) {
 				revs->verbose_header = 1;
 				get_commit_format(arg+8, revs);
 				continue;
 			}
+			if (!prefixcmp(arg, "--pretty=")) {
+				revs->verbose_header = 1;
+				get_commit_format(arg+9, revs);
+				continue;
+			}
 			if (!strcmp(arg, "--graph")) {
 				revs->topo_order = 1;
 				revs->rewrite_parents = 1;