Made the finishing of release/hotfix branches fail-safe. When a
release/hotfix branch fails finishing, the user may just try so again.
diff --git a/gitflow-common b/gitflow-common
index c96b845..4f2a85a 100644
--- a/gitflow-common
+++ b/gitflow-common
@@ -148,6 +148,10 @@
 	fi
 }
 
+gitflow_tag_exists() {
+	has $1 $ALL_TAGS
+}
+
 #
 # gitflow_test_branches_equal()
 #