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.
Ancestors of 5
4 2 0
Ancestors of 6 and 5
3 4 2 1 0
Ancestors of 5 and 4
4 2 0
Descendants of 5
7 8
Descendants of 5 and 3
6 7 8
Descendants of 5 and 4
5 7 8