tests: make test-convert-bzr.t more stable

The test would occasionally fail because datesort don't have sub-second
granularity and thus can't sort commits made in the same second.

The test is made more stable by adding 1 second of sleep to make sure the bzr
commits are done with different timestamps.

tests/test-convert-bzr.t

@@ -225,6 +225,7 b' Multiple branches'
   $ bzr switch -b branch
   Tree is up to date at revision 1.
   Switched to branch: *repo/branch/ (glob)
+  $ sleep 1
   $ echo b > b
   $ bzr add -q b
   $ bzr ci -qm addb
@@ -233,6 +234,7 b' Multiple branches'
   $ bzr switch --force ../repo/trunk
   Updated to revision 1.
   Switched to branch: */repo/trunk/ (glob)
+  $ sleep 1
   $ echo a >> a
   $ bzr ci -qm changea
   $ cd ..