largefiles: fix confusion upon removal of added largefile (issue3176)
This patch makes "hg remove" work the same way on largefiles as it does on
regular Mercurial files. If you try to remove an added largefile, the removal
fails and you are instead prompted to use "hg forget" to undo the add.
$ hg init repo
$ cd repo
$ touch foo
$ hg ci -Am 'add foo'
adding foo
$ hg up -C null
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
this should be stored as a delta against rev 0
$ echo foo bar baz > foo
$ hg ci -Am 'add foo again'
adding foo
created new head
$ hg debugindex foo
rev offset length base linkrev nodeid p1 p2
0 0 0 0 0 b80de5d13875 000000000000 000000000000
1 0 24 0 1 0376abec49b8 000000000000 000000000000