nix-prefetch-git: add -h as shortcut for --help to improve quality of life

Especially as a new user it is a much better experience to receive a
proper help response to `-h`. Currently passing `-h` will cause some
runtime error with the `git remote` error help being shown. Not very
helpful.

It doesn't hurt to be a bit more user friendly in this case.

pkgs/build-support/fetchgit/nix-prefetch-git

@@ -68,7 +68,7 @@ for arg; do
             --leave-dotGit) leaveDotGit=true;;
             --fetch-submodules) fetchSubmodules=true;;
             --builder) builder=true;;
-            --help) usage; exit;;
+            -h|--help) usage; exit;;
             *)
                 : $((++argi))
                 case $argi in