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/default.nix

@@ -13,6 +13,7 @@
 in
 { url, rev ? "HEAD", md5 ? "", sha256 ? "", leaveDotGit ? deepClone
 , fetchSubmodules ? true, deepClone ? false
+, branchName ? null
 , name ? urlToName url rev
 }:
 
@@ -51,7 +52,7 @@ stdenv.mkDerivation {
   outputHashMode = "recursive";
   outputHash = if sha256 == "" then md5 else sha256;
 
-  inherit url rev leaveDotGit fetchSubmodules deepClone;
+  inherit url rev leaveDotGit fetchSubmodules deepClone branchName;
 
   GIT_SSL_CAINFO = "${cacert}/etc/ca-bundle.crt";