fish: functions/gm: Automatically detect master or main and merge

git merge master/main is a frequent enough operation. Automatically
detect whether the primary branch is master or main and merge.

fish/.config/fish/functions/gm.fish

@@ -1,3 +1,8 @@
 function gm
-    git merge $argv
+    if count $argv >/dev/null
+        git merge $argv
+    else
+        set -l remote (git symbolic-ref refs/remotes/origin/HEAD | awk -F'/' '{print $4}')
+        git merge origin/$remote
+    end
 end