localrepo: don't add deleted files to list of modified/added files (
issue2761)
If a file is deleted (rm, not 'hg rm') from the working dir
an attempt to run 'hg diff -r X', with the file being present in X will
cause an abort.
We didn't check if the file has been deleted from the working dir
and later on tried to open it to compare with the one from X, causing the abort.
This fix adds that check. Consequently, no output will be returned.