format-patch: give --reroll-count a short synonym -v

Accept "-v" as a synonym to "--reroll-count", so that users can say
"git format-patch -v4 master", instead of having to fully spell it
out as "git format-patch --reroll-count=4 master".

As I do not think of a reason why users would want to tell the
command to be "verbose", I think this should be OK.

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

t/t4014-format-patch.sh

@@ -245,6 +245,14 @@ test_expect_success 'reroll count' '
 	! grep -v "^Subject: \[PATCH v4 [0-3]/3\] " subjects
 '
 
+test_expect_success 'reroll count (-v)' '
+	rm -fr patches &&
+	git format-patch -o patches --cover-letter -v4 master..side >list &&
+	! grep -v "^patches/v4-000[0-3]-" list &&
+	sed -n -e "/^Subject: /p" $(cat list) >subjects &&
+	! grep -v "^Subject: \[PATCH v4 [0-3]/3\] " subjects
+'
+
 check_threading () {
 	expect="$1" &&
 	shift &&