Tag compatibility. Resolves #1487 (#1492)

* Add --global-tag-lookup

* Add bin/gollum-migrate-tags

* Add migration tests (not on Travis)

test/examples/lotr_migration.git/hooks/pre-receive.sample

@@ -0,0 +1,24 @@
+#!/bin/sh
+#
+# An example hook script to make use of push options.
+# The example simply echoes all push options that start with 'echoback='
+# and rejects all pushes when the "reject" push option is used.
+#
+# To enable this hook, rename this file to "pre-receive".
+
+if test -n "$GIT_PUSH_OPTION_COUNT"
+then
+	i=0
+	while test "$i" -lt "$GIT_PUSH_OPTION_COUNT"
+	do
+		eval "value=\$GIT_PUSH_OPTION_$i"
+		case "$value" in
+		echoback=*)
+			echo "echo from the pre-receive-hook: ${value#*=}" >&2
+			;;
+		reject)
+			exit 1
+		esac
+		i=$((i + 1))
+	done
+fi