mailinfo: Do not use -u=<encoding>; say --encoding=<encoding>

Specifying the value for a single letter, single dash option
parameter with equal sign looked funny, and more importantly
calling the flag to override encoding from utf-8 to something
else "-u" (obviously abbreviated from "utf-8") did not make any
sense.  So spell it out.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

Documentation/git-mailinfo.txt

@@ -8,7 +8,7 @@ git-mailinfo - Extracts patch from a single e-mail message.
 
 SYNOPSIS
 --------
-'git-mailinfo' [-k] [-u] <msg> <patch>
+'git-mailinfo' [-k] [-u | --encoding=<encoding>] <msg> <patch>
 
 
 DESCRIPTION
@@ -37,10 +37,17 @@ OPTIONS
 	author email are taken from the e-mail without any
 	charset conversion, after minimally decoding MIME
 	transfer encoding.  This flag causes the resulting
-	commit to be encoded in utf-8 by transliterating them.
+	commit to be encoded in the encoding specified by
+	i18n.commitencoding configuration (defaults to utf-8) by
+	transliterating them. 
 	Note that the patch is always used as is without charset
 	conversion, even with this flag.
 
+--encoding=<encoding>::
+	Similar to -u but if the local convention is different
+	from what is specified by i18n.commitencoding, this flag
+	can be used to override it.
+
 <msg>::
 	The commit log message extracted from e-mail, usually
 	except the title line which comes from e-mail Subject.

mailinfo.c

@@ -717,7 +717,7 @@ static void handle_body(void)
 }
 
 static const char mailinfo_usage[] =
-	"git-mailinfo [-k] [-u] msg patch <mail >info";
+	"git-mailinfo [-k] [-u | --encoding=<encoding>] msg patch <mail >info";
 
 int main(int argc, char **argv)
 {
@@ -731,8 +731,8 @@ int main(int argc, char **argv)
 			keep_subject = 1;
 		else if (!strcmp(argv[1], "-u"))
 			metainfo_charset = git_commit_encoding;
-		else if (!strncmp(argv[1], "-u=", 3))
-			metainfo_charset = argv[1] + 3;
+		else if (!strncmp(argv[1], "--encoding=", 11))
+			metainfo_charset = argv[1] + 11;
 		else
 			usage(mailinfo_usage);
 		argc--; argv++;