overlayworkingctx: don't include added-then-deleted files in memctx

Authored by martinvonz.

Description

overlayworkingctx: don't include added-then-deleted files in memctx

If a file (such as a .orig file) is temporarily added to the
overlayworkingctx and then deleted, it's still going to be in the
_cache dict. In tomemctx(), we created the list of files from
_cache.keys(), so the memctx.files() would include the temporary
file. That was fine because the list of files was only used in
localrepo.commitctx() (I think), where there's an extra filtering of
incorrectly removed files (annotated with an inaccurate "update
manifest" comment). I'd like to call memctx.files() in another case,
but first we need to make it accurate.

Differential Revision: https://phab.mercurial-scm.org/D6361