fast-export: add --signed-tags=warn-strip mode

This issues a warning while stripping signatures from signed tags, which
allows us to use it as default behaviour for remote helpers which cannot
specify how to handle signed tags.

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t9350-fast-export.sh

@@ -146,6 +146,12 @@ test_expect_success 'signed-tags=strip' '
 
 '
 
+test_expect_success 'signed-tags=warn-strip' '
+	git fast-export --signed-tags=warn-strip sign-your-name >output 2>err &&
+	! grep PGP output &&
+	test -s err
+'
+
 test_expect_success 'setup submodule' '
 
 	git checkout -f master &&