fix(git): silently fail in `git_main_branch` if not in a git repo (#9484)

Co-authored-by: Marc Cornellà <marc.cornella@live.com>

plugins/git/git.plugin.zsh

@@ -31,6 +31,7 @@ function work_in_progress() {
 
 # Check if main exists and use instead of master
 function git_main_branch() {
+  command git rev-parse --git-dir &>/dev/null || return
   local branch
   for branch in main trunk; do
     if command git show-ref -q --verify refs/heads/$branch; then