Update can-ff-merge to use older git branch name

Update can-ff-merge alias to use older git branch name technique instead
of git branch --show-current, so we can support Git before version
2.22.0

Resolves #1

Author: salcode

gitconfig

@@ -2,7 +2,7 @@
 	can-ff-merge = "!f() { \
 		[ -e "${1}" ] && echo "Missing branch name to merge" && exit 1; \
 		git "merge-base --is-ancestor \
-		$(git branch --show-current) ${1}" \
+		$(git rev-parse --abbrev-ref HEAD) ${1}" \
 		&& echo 'Yes, this can ff-merge' && exit 0 \
 		|| echo 'No, this can NOT ff-merge' && exit 1; \
 	}; f"