nodemap: double check the source docket when doing incremental update
In theory, the index will have the information we expect it to have. However by
security, it seems safer to double check that the incremental data are generated
from the data currently on disk.
Differential Revision: https://phab.mercurial-scm.org/D7890
$ hg init
$ echo This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ ls
a
$ echo This is file b1 > b
$ hg add b
$ hg commit -m "commit #1"
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
B should disappear
$ ls
a