Add --format that is a synonym to --pretty

Some people prefer to call the pretty-print styles "format", and get
annoyed to see "git log --format=short" fail.  Introduce it as a synonym
to --pretty so that both can be used.

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

Documentation/pretty-options.txt

@@ -1,4 +1,5 @@
 --pretty[='<format>']::
+--format[='<format>']::
 
 	Pretty-print the contents of the commit logs in a given format,
 	where '<format>' can be one of 'oneline', 'short', 'medium',

revision.c

@@ -1144,7 +1144,7 @@ static int handle_revision_opt(struct rev_info *revs, int argc, const char **arg
 	} else if (!strcmp(arg, "--pretty")) {
 		revs->verbose_header = 1;
 		get_commit_format(arg+8, revs);
-	} else if (!prefixcmp(arg, "--pretty=")) {
+	} else if (!prefixcmp(arg, "--pretty=") || !prefixcmp(arg, "--format=")) {
 		revs->verbose_header = 1;
 		get_commit_format(arg+9, revs);
 	} else if (!strcmp(arg, "--graph")) {