Support installing repo from URL with non-master branch

[tennox]

Somewhat related to #905 and #685, but by use-case is not a package in packages-main repo, but a GitLab repo, for which install currently works fine, but omf update results in this:
(on omf channel dev)

Oh My Fish is up to date.
You are now using Oh My Fish version 7-44-gd428b72.
Updating https://github.com/oh-my-fish/packages-main master... Done!
fatal: couldn't find remote ref refs/heads/master
Could not update fish-functions.
bobthefish is already up-to-date.
default is already up-to-date.

While debugging I found set branch "master" in pkg/omf/functions/repo/omf.repo.pull.fish and changed it to use current branch as fallback/default an old convention

I tried thinking of all the edge-cases you're covering, but not sure if I understood correctly.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• New and existing tests pass locally with my changes

[tennox]

For anyone who wants to try:

omf channel dev
cd $OMF_PATH
git remote set-url origin https://github.com/tennox/oh-my-fish.git
git pull origin master
omf update

[scorphus]

Thanks, @tennox!