extdiff: copy back files to the working directory if the size changed
In theory, it should be enough to pay attention only to the modification time
when detecting if a snapshotted working directory file changed. In practice,
BeyondCompare preserves all file attributes when syncing files at the directory
level. (If you open the file and sync individual hunks, then mtime does change,
and everything was being copied back as desired.) I'm not sure how many other
synchronization tools would trigger this issue, but it's annoyingly inconsistent
(if a single file is diffed, it isn't snapshotted, so the same BeyondCompare
file sync operation _is_ visible, because wdir() is updated in place.
I filed a bug with them, and they stated it is on their wish list, but won't be
fixed in the near term. This isn't a complete fix (there is still the case of
the size not changing), but this seems like a trivial enough change to fix most
of the problem. I suppose we could fool around with making files in the other
snapshot readonly, and copy back if we see the readonly bit copied. That seems
pretty hacky though, and only works if the external tool copies all attributes.