nix-prefetch-git: don't fetch everything when given a hash

It's hugely inefficient as we can't use shallow cloning (--depth=1).

This has been tested and adapted for quite a few hosts fetchgit is used on in
Nixpkgs. For those where fetching the hash directly doesn't work (most notably
git.savannah.gnu.org), we simply fall back to the old method.

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

@@ -140,7 +140,7 @@ url_to_name(){
     fi
 }
 
-# Fetch everything and checkout the right sha1
+# Fetch and checkout the right sha1
 checkout_hash(){
     local hash="$1"
     local ref="$2"
@@ -149,6 +149,7 @@ checkout_hash(){
         hash=$(hash_from_ref "$ref")
     fi
 
+    clean_git fetch ${builder:+--progress} --depth=1 origin "$hash" || \
     clean_git fetch -t ${builder:+--progress} origin || return 1
 
     local object_type=$(git cat-file -t "$hash")