Look for branches that are literally the word main

I have a repo with many other branches that contain the string `main`,
but our default branch is still master.  This grep was seeing those other branches
and deciding that my default branch was `main`.  This tighter regex
fixes that behavior for me.

aliases/available/git.aliases.bash

@@ -199,7 +199,7 @@ function gdv() {
 }
 
 function get_default_branch() {
-	if git branch | grep -q main; then
+	if git branch | grep -q '^main$'; then
 		echo main
 	else
 		echo master