nix-prefetch-git: Run git-init with --initial-branch=master

The reason for this change is simply to avoid the following messages
that are unnecessary and can be confusing (and these messages will be
repeated for each submodule):
hint: Using 'master' as the name for the initial branch. This default branch name
hint: is subject to change. To configure the initial branch name to use in all
hint: of your new repositories, which will suppress this warning, call:
hint:
hint:   git config --global init.defaultBranch <name>
hint:
hint: Names commonly chosen instead of 'master' are 'main', 'trunk' and
hint: 'development'. The just-created branch can be renamed via this command:
hint:
hint:   git branch -m <name>

With this change the behaviour remains unchanged (apart from the
suppressed "warning" in the console output of course) and therefore this
doesn't cause any hashes to change and by default nix-prefetch-git uses
the "fetchgit" branch anyway (branchName can be set to override the
default):
Switched to a new branch 'fetchgit'

For that reason the initial branch name doesn't matter anyway and since
we're not relying on / hardcoding "master" we could simply switch to
"main" (which seems most common nowadays). See [0] for more details on
why this wouldn't break anything.
However, since the initial branch name doesn't matter and to avoid any
additional risks it was "decided" to keep using "master" (s. #113313).

[0]: https://github.com/NixOS/nixpkgs/pull/113313#issuecomment-780589516

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

@@ -105,7 +105,7 @@ fi
 
 init_remote(){
     local url=$1
-    clean_git init
+    clean_git init --initial-branch=master
     clean_git remote add origin "$url"
     ( [ -n "$http_proxy" ] && clean_git config http.proxy "$http_proxy" ) || true
 }