Add a --no-check-device option to bup index (cf. tar).

Ignore stat.st_dev when comparing files against existing index entries, like
git does without #define USE_STDEV:

  http://www.kernel.org/pub/software/scm/git/docs/v1.7.10.1/technical/racy-git.txt

See also: "info tar".

Signed-off-by: Zak Wilcox <iwilcox@iwilcox.me.uk>
[rlb@defaultvalue.org: change original --ignore-dev to --no-check-device to match tar.
 Adjust code to work with current master.
 Remove tests -- will be reintroduced shortly.]
Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Add index --no-check-device to the index manpage.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Add tests for index --no-check-device, and support for t/mnt.

Add support for t/mnt; anything mounted there will be cleaned up
during "make clean".

Thanks to Zak Wilcox <iwilcox@iwilcox.me.uk> for an earlier version of
these tests.

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