helpers.py: check subprocess exit status in readpipe().

Checking the exit status seems like a good idea.  Note that we can't
just use check_output() because it wasn't added until Python 2.7.

Thanks to Alexander Barton <alex@barton.de> for reporting the version
incompatibility with respect to an earlier version of this patch.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Use the author date, not committer date for the bup save date.

Since it's the author date that indicates when the save was made, use
that instead of the committer date to represent saves.

Doing so means that any operations on branches (like rebase,
cherry-pick, etc.) won't change the "save date" as far as bup is
concerned.

This should have no effect with respect to backward compatibility
because until now, bup always used the same date for the author and
committer.

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