gpg-interface: refactor 'enum sign_mode' parsing

The definition of 'enum sign_mode' as well as its parsing code are in
"builtin/fast-export.c". This was fine because `git fast-export` was the
only command with '--signed-tags=<mode>' or '--signed-commits=<mode>'
options.

In a following commit, we are going to add a similar option to `git
fast-import`, which will be simpler, easier and cleaner if we can reuse
the 'enum sign_mode' defintion and parsing code.

So let's move that definition and parsing code from
"builtin/fast-export.c" to "gpg-interface.{c,h}".

While at it, let's fix a small indentation issue with the arguments of
parse_opt_sign_mode().

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

gpg-interface.h

@@ -104,4 +104,19 @@ int check_signature(struct signature_check *sigc,
 void print_signature_buffer(const struct signature_check *sigc,
 			    unsigned flags);
 
+/* Modes for --signed-tags=<mode> and --signed-commits=<mode> options. */
+enum sign_mode {
+	SIGN_ABORT,
+	SIGN_WARN_VERBATIM,
+	SIGN_VERBATIM,
+	SIGN_WARN_STRIP,
+	SIGN_STRIP,
+};
+
+/*
+ * Return 0 if `arg` can be parsed into an `enum sign_mode`. Return -1
+ * otherwise.
+ */
+int parse_sign_mode(const char *arg, enum sign_mode *mode);
+
 #endif