git: Dynamically identify the "default" branch

This broadly assumes that the "default" remote is called "origin", but
that's better than assuming that plus the default branch name.

Thanks to https://joshtronic.com/2020/08/09/how-to-get-the-default-git-branch/

Suggested-by: Chris Lane <chris@chrislane.com>
Signed-off-by: Joe Groocock <me@frebib.net>

git/config

@@ -37,6 +37,7 @@
 	count = !echo $(pwd) && git rev-list HEAD --count
 	unpushed = !git log --oneline @{u}..HEAD
 	leaders = !git shortlog -s -n --all --no-merges
+	defbranch = !git symbolic-ref refs/remotes/origin/HEAD | sed \"s@^refs/remotes/origin/@@\"
 [advice]
 	statusHints = false
 	pushUpdateRejected = false