Document accommodations for filesystems with low-resolution timestamps.

...and some effects thereof.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Store metadata in the index, in bupindex.meta; only store unique values.

See DESIGN for more information.

Update the index format header to 'BUPI\0\0\0\5' (version 5).

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Add the filename/fd and invalid value to stat_struc_to_py error messages.

Thanks to Nathan Bird <ecthellion@gmail.com> for the initial report.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Remove the initial "bup tick" from index.

I'm the one who originally added this, and after working on the
support for index metadata, I'm fairly confident that I misunderstood
the situation, and that the relevant cases are covered by the existing
code (via tmax, etc.).

Signed-off-by: Rob Browning <rlb@defaultvalue.org>