t7004: factor out gpg setup

Other test scripts may want to look at or verify signed
tags, and the setup is non-trivial. Let's factor this out
into lib-gpg.sh for other tests to use.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/lib-gpg.sh

@@ -0,0 +1,26 @@
+#!/bin/sh
+
+gpg_version=`gpg --version 2>&1`
+if test $? = 127; then
+	say "You do not seem to have gpg installed"
+else
+	# As said here: http://www.gnupg.org/documentation/faqs.html#q6.19
+	# the gpg version 1.0.6 didn't parse trust packets correctly, so for
+	# that version, creation of signed tags using the generated key fails.
+	case "$gpg_version" in
+	'gpg (GnuPG) 1.0.6'*)
+		say "Your version of gpg (1.0.6) is too buggy for testing"
+		;;
+	*)
+		# key generation info: gpg --homedir t/lib-gpg --gen-key
+		# Type DSA and Elgamal, size 2048 bits, no expiration date.
+		# Name and email: C O Mitter <committer@example.com>
+		# No password given, to enable non-interactive operation.
+		cp -R "$TEST_DIRECTORY"/lib-gpg ./gpghome
+		chmod 0700 gpghome
+		GNUPGHOME="$(pwd)/gpghome"
+		export GNUPGHOME
+		test_set_prereq GPG
+		;;
+	esac
+fi