Give short-hands to --pretty=tformat:%formatstring

Allow --pretty="%h %s" (and --format="%h %s") as shorthand for an often
used option --pretty=tformat:"%h %s".

Signed-off-by: Nanako Shiraishi <nanako3@lavabit.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/pretty-formats.txt

@@ -152,3 +152,12 @@ $ git log -2 --pretty=tformat:%h 4da45bef \
 4da45be
 7134973
 ---------------------
++
+In addition, any unrecognized string that has a `%` in it is interpreted
+as if it has `tformat:` in front of it.  For example, these two are
+equivalent:
++
+---------------------
+$ git log -2 --pretty=tformat:%h 4da45bef
+$ git log -2 --pretty=%h 4da45bef
+---------------------

pretty.c

@@ -10,6 +10,15 @@
 
 static char *user_format;
 
+static void save_user_format(struct rev_info *rev, const char *cp, int is_tformat)
+{
+	free(user_format);
+	user_format = xstrdup(cp);
+	if (is_tformat)
+		rev->use_terminator = 1;
+	rev->commit_format = CMIT_FMT_USERFORMAT;
+}
+
 void get_commit_format(const char *arg, struct rev_info *rev)
 {
 	int i;
@@ -33,12 +42,7 @@ void get_commit_format(const char *arg, struct rev_info *rev)
 		return;
 	}
 	if (!prefixcmp(arg, "format:") || !prefixcmp(arg, "tformat:")) {
-		const char *cp = strchr(arg, ':') + 1;
-		free(user_format);
-		user_format = xstrdup(cp);
-		if (arg[0] == 't')
-			rev->use_terminator = 1;
-		rev->commit_format = CMIT_FMT_USERFORMAT;
+		save_user_format(rev, strchr(arg, ':') + 1, arg[0] == 't');
 		return;
 	}
 	for (i = 0; i < ARRAY_SIZE(cmt_fmts); i++) {
@@ -50,6 +54,10 @@ void get_commit_format(const char *arg, struct rev_info *rev)
 			return;
 		}
 	}
+	if (strchr(arg, '%')) {
+		save_user_format(rev, arg, 1);
+		return;
+	}
 
 	die("invalid --pretty format: %s", arg);
 }