fetchgit: Add support for specifying branch name

This is useful when `leaveDotGit = true` and some other derivation
expects some branch name to exist.

Previously, `nix-prefetch-git` always created a branch with a
hard-coded name (`fetchgit`).

pkgs/build-support/fetchgit/builder.sh

@@ -9,6 +9,7 @@ header "exporting $url (rev $rev) into $out"
 $fetcher --builder --url "$url" --out "$out" --rev "$rev" \
   ${leaveDotGit:+--leave-dotGit} \
   ${deepClone:+--deepClone} \
-  ${fetchSubmodules:+--fetch-submodules}
+  ${fetchSubmodules:+--fetch-submodules} \
+  ${branchName:+--branch-name "$branchName"}
 
 stopNest