Support installing repo from URL with non-master branch

Somewhat related to #905 and #685

pkg/omf/functions/repo/omf.repo.pull.fish

@@ -1,12 +1,12 @@
 function omf.repo.pull -a repo_dir branch
-  if test -z "$branch"
-    set branch "master"
-  end
-
   function __omf.repo.git -V repo_dir
     command git -C "$repo_dir" $argv
   end
 
+  if test -z "$branch"
+    set branch (__omf.repo.git symbolic-ref -q --short HEAD)
+  end
+
   set -l remote origin
   if test (__omf.repo.git config --get remote.upstream.url)
     set remote upstream