Fix ArrayIndexOutOfBounds on non-square exact rename matrix

Fix ArrayIndexOutOfBounds on non-square exact rename matrix

If the exact rename matrix for a particular ObjectId isn't square we
crashed with an ArrayIndexOutOfBoundsException because the matrix
entries were encoded backwards.  The encode function accepts the
source (aka deleted) index first, not second.  Add a unit test to
cover this non-square case to ensure we don't have this regression
in the future.

Change-Id: I5b005e5093e1f00de2e3ec104e27ab6820203566
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>