feat(git): accept `default` and `mainline` as main branches (#11431)

plugins/git/git.plugin.zsh

@@ -31,7 +31,7 @@ function work_in_progress() {
 function git_main_branch() {
   command git rev-parse --git-dir &>/dev/null || return
   local ref
-  for ref in refs/{heads,remotes/{origin,upstream}}/{main,trunk}; do
+  for ref in refs/{heads,remotes/{origin,upstream}}/{main,trunk,mainline,default}; do
     if command git show-ref -q --verify $ref; then
       echo ${ref:t}
       return