In indirect mode, now checks the inode cache to detect changes to a file.
Note that a file can still be changed if a process has it open for write,
after landing in the annex.
In direct mode, some checking of the inode cache was done before, but
from a much later point, so fewer modifications could be detected. Now it's
as good as indirect mode.
On crippled filesystems, no lock down is done before starting to add a
file, so checking the inode cache is the only protection we have.
7ce30b534f