avoid removing old associated file when there is a merge conflict

avoid removing old associated file when there is a merge conflict

It makes sense to keep the key used by the old version of an
associated file, until the merge conflict is resolved.

Note that, since in this case git diff is being run with --index, it's
not possible to use -1 or -3, which would let the keys
associated with the new versions of the file also be added. That would
be better, because it's possible that the local modification to the file
that caused the merge conflict has not yet gotten its new key recorded
in the db.

Opened a bug about a case this is thus not able to address.

Sponsored-by: Boyd Stephen Smith Jr. on Patreon