dirstate: don't fail when dropping a not-tracked file (issue3080)
Complex merges with divergent renames can cause a file to be 'moved'
twice, causing dirstate.drop() to be called twice. Rather than try to
ensure there are no unexpected corner cases where this can happen, we
simply ignore drops of files that aren't tracked.
$ hg init
$ echo 123 > a
$ hg add a
$ hg commit -m "first" a
$ mkdir sub
$ echo 321 > sub/b
$ hg add sub/b
$ hg commit -m "second" sub/b
$ cat sub/b
321
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ cat sub/b 2>/dev/null || echo "sub/b not present"
sub/b not present
$ test -d sub || echo "sub not present"
sub not present