metadataonlyctx: don't crash when reusing the manifest with deletions
This was originally fixed by Mateusz Kwapich for the `metaedit` command in
fb-hgext with a test for the `metaedit` command. It didn't get upstreamed
because `metaedit` was not in core.
This patch fixes the crash and adds a test about `metadataonlyctx` to
avoid future regressions.
Differential Revision: https://phab.mercurial-scm.org/D550
Test issue2761
$ hg init
$ touch to-be-deleted
$ hg add
adding to-be-deleted
$ hg ci -m first
$ echo a > to-be-deleted
$ hg ci -m second
$ rm to-be-deleted
$ hg diff -r 0
Same issue, different code path
$ hg up -C
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ touch does-not-exist-in-1
$ hg add
adding does-not-exist-in-1
$ hg ci -m third
$ rm does-not-exist-in-1
$ hg diff -r 1