log: give --[no-]use-mailmap a more sensible synonym --[no-]mailmap

The option name "--use-mailmap" looks OK, but it becomes awkward
when you have to negate it, i.e. "--no-use-mailmap".  I, perhaps
with many other users, always try "--no-mailmap" and become unhappy
to see it fail.

Add an alias "--[no-]mailmap" to remedy this.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-log.txt

@@ -49,6 +49,7 @@ OPTIONS
 	Print out the ref name given on the command line by which each
 	commit was reached.
 
+--[no-]mailmap::
 --[no-]use-mailmap::
 	Use mailmap file to map author and committer names and email
 	addresses to canonical real names and email addresses. See

builtin/log.c

@@ -173,6 +173,7 @@ static void cmd_log_init_finish(int argc, const char **argv, const char *prefix,
 		OPT__QUIET(&quiet, N_("suppress diff output")),
 		OPT_BOOL(0, "source", &source, N_("show source")),
 		OPT_BOOL(0, "use-mailmap", &mailmap, N_("Use mail map file")),
+		OPT_ALIAS(0, "mailmap", "use-mailmap"),
 		OPT_STRING_LIST(0, "decorate-refs", &decorate_refs_include,
 				N_("pattern"), N_("only decorate refs that match <pattern>")),
 		OPT_STRING_LIST(0, "decorate-refs-exclude", &decorate_refs_exclude,