scmutil: rename filecacheentry to filecachesubentry
Upcoming patches will allow the file cache to watch over multiple files, and
call the decorated function again if any of the files change.
The particular use case for this is the bookmark store, which needs to be
invalidated if either .hg/bookmarks or .hg/bookmarks.current changes. (This
doesn't currently happen, which is a bug. This bug will also be fixed in
upcoming patches.)
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.