remotefilelog: fix crash on `hg addremove` of added-but-deleted file
If you `hg add` a file and then delete it from disk, and then run `hg
addremove`, the file ends up in the "removed" set that gets passed to
the findrenames() override. We then crash because the file is not in
the working copy parent. This patch fixes that.
Differential Revision: https://phab.mercurial-scm.org/D6194