rm-between-index-and-save.sh: fix race condition.

Fix a timing issue that often caused the test to fail -- the (removed)
file "foo" was marked as invalid in the index when it shouldn't have
been because it was modified less than tmax before the index run.

Insert a "bup tick" just before "bup index" so that foo won't be
marked invalid by the second run.

Don't expect the second "bup save" to fail since foo will now be valid
in the index (and hence ignored), even though it is no longer
available.  The removal will be noticed by the next index+save.

When removing directories, the outcome is different: since metadata is
saved per directory, "bup save" notices that the directory is missing
and logs an "errno 2" error, but does not crash, which is what the
tested patch is concerned with.  A third index+save run completes
without error.

This also means that without a "bup tick" before an index run, calls,
"bup save" might exit with non-zero exitcode when files/directories
are removed after bup index, but bup is not guaranteed to notice
missing files.

Signed-off-by: Tim Riemenschneider <git@tim-riemenschneider.de>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>