[FEATURE REQUEST] Update only a specific branch with fileserver.update

[FredrikLiindgren]

Is your feature request related to a problem? Please describe.
I'm frustrated when I push custom module/state/runner changes to a branch and have to update the entire remote if I want to speed up the synchronization of changes.

Describe the solution you'd like
I want to be able to just update a single branch on a remote with fileserver.update.

salt/salt/runners/fileserver.py

Line 322 in a686ce0

def update(backend=None, **kwargs):

Something like this: salt-run fileserver.update backend=gitfs remote=my-remote branch=my-branch

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[OrangeDog]

That's not really going to improve anything. It's very unlikely that you have so many changes on other branches that there will be a noticeable difference. Just typing the option will probably take longer.