fix: allow fixing untracked files when given as arguments
It's more useful to fix the file than to silently avoid it when the user does
hg fix --working-dir untracked-file
We will still do nothing to ignored files, even if they are specified. This may
be safer, given that such files can unexpectedly slip into the arguments via a
shell glob or fileset.
Differential Revision: https://phab.mercurial-scm.org/D6165