An earlier commit (mislabeled) made direct mode fsck check file checksums.
While it's expected for files to change at any time in direct mode, and so
fsck cannot complain every time there's a checksum mismatch, it is possible
for it to detect when a file does not *seem* to have changed, then check
its checksum, and so detect disk corruption or other problems.
This commit improves that, by checking a second time, if the checksum
fails, that the file is still not modified, before taking action. This way,
a direct mode file can be modified while being fscked.
Sometimes it seems that git-cat-file --batch stops getting info for
files in the current repo, when ":file" is fed to it. I have not reproduced
this at the command line, but only when using git annex whereis and git
annex move inside a direct mode repo. Those failed, because cat-file
returned "file missing". OTOH, git annex find works fine, despite passing
the same file to cat-file. It seems that the failing commands first asked
cat-file to show a file on the git-annex branch. Perhaps it got "stuck" on
that branch? But I cannot repoduce it running cat-file by hand. Most
strange. HEAD is a workaround for this extreme weirdness, since I spent a
good 2 hours struggling with it already.
The expensive scan uses lookupFile, but in direct mode, that doesn't work
for files that are present. So the scan was not finding things that are
present that need to be uploaded. (It did find things not present that
needed to be downloaded.)
Now lookupFile also works in direct mode. Note that it still prefers
symlinks on disk to info committed to git, in direct mode. This is
necessary to make things like Assistant.Threads.Watcher.onAddSymlink
work correctly, when given a new symlink not yet checked into git (or
replacing a file checked into git).
