fs/fs.go: extract func existsAlternate()
fs/fs.go: extract func existsAlternate()

In a subsequent commit, we will support reading a list of ODB alternates
from the OS environment variable, "$GIT_ALTERNATE_OBJECT_DIRECTORIES".

To prepare for this, let's extract a function that determines whether or
not a given string is suitable as an alternates directory.

github/git-lfs,github/git-lfs,github/git-lfs