browse-remote-prefer-symbolic does not fallback to git revision in detached HEAD state

[gagbo]

Thanks for the package :)

This let-binding doesn't fall back nicely to the git revision if we're in detached HEAD but still prefering the symbolic name when available.

It would be better to try and get the git revision when vc-git-symbolic-ref returns nil, or maybe change the name of the variable to explicitely tell that "prefer" is actually "force-or-nil".