Make git a little quieter

On my system whenever I run the tests without specifying an initial branch name,
I get a lot of messages about setting the default branch name globally. This is
just noise for these tests, so this specifies a branch name manually to make it
go away.

The name ‘gac-test’ was chosen to make it clear that we're dealing with a
repository created specifically for ‘git-auto-commit-mode’ in case somehow a
repository gets left behind.

tests/git-auto-commit-mode-tests.el

@@ -38,7 +38,7 @@
     (setq temp-dir (make-temp-file "gac-" t)
           current-directory default-directory
           default-directory temp-dir)
-    (shell-command "git init")
+    (shell-command "git init --initial-branch=gac-test")
     (shell-command "git config user.email user@example.com")
     (shell-command "git config user.name \"User Example\""))