git: Add an init template, use it to create the main branch

See
56d614f806.

> This change adds a HEAD file to the Git template with a different ref.
This means that new repositories will commit to a "main" branch by
default instead.

tag-zsh/zsh/configs/git.zsh

@@ -1,13 +1,7 @@
 # No arguments: `git status`
 # With arguments: acts like `git`
 g() {
-  if [[ $1 == init ]];
-  then
-    # Prefer "main" to "master"
-    git "$@"
-    git checkout -b main
-    git branch -d master
-  elif [[ $# > 0 ]];
+  if [[ $# > 0 ]];
   then
     git $@
   else