largefiles: handle that a found standin file doesn't exist when removing it

I somehow ended up in a situation where hg crashed on an unlink I introduced in
328545c7d8a1.

I don't know how it happened and can't reproduce it. It seems like it only can
happen when the file is removed between the time of check in a working
directory context walk that finds a standin file, and the time of use when we
try to remove it because the corresponding largefile doesn't exist.

But better safe than sorry: replace the plain unlink with unlinkpath with
ignoremissing=True. That will also remove remaining empty directories, which
arguably is more correct.

hgext/largefiles/reposetup.py

@@ -217,7 +217,7 b' def reposetup(ui, repo):'
                             # standin. Removing a file as a side effect of
                             # running status is gross, but the alternatives (if
                             # any) are worse.
-                            self.wvfs.unlink(standin)
+                            self.wvfs.unlinkpath(standin, ignoremissing=True)
 
                     # Filter result lists
                     result = list(result)