I hope this doesn't impact speed much -- it does have to pull out a value
from Annex state every time it accesses the branch now.
The test case I dropped has never caught any problems that I can remember,
and would have been rather difficult to convert.
Having one module that knows about all the filenames used on the branch
allows working back from an arbitrary filename to enough information about
it to implement dropping dead remotes and doing other log file compacting
as part of a forget transition.
Avoid ever using read to parse a non-haskell formatted input string.
show :: Key is arguably still show abuse, but displaying Keys as filenames
is just too useful to give up.
Checks location log information, and file contents.
Does not check that numcopies is satisfied, as .gitattributes information
about numcopies is not available in a bare repository. In practice, that
should not be a problem, since fsck is also run in a checkout and will
check numcopies there.
