t/lib-bitmap.sh: parameterize tests over reverse index source

t/lib-bitmap.sh: parameterize tests over reverse index source

To prepare for reading the reverse index data out of the MIDX itself,
teach the `test_rev_exists` function to take an expected "source" for
the reverse index data.

When given "rev", it asserts that the MIDX's `.rev` file exists, and is
loaded when verifying the integrity of its bitmaps. Otherwise, it
ensures that trace2 reports the source of the reverse index data as the
same string which was given to test_rev_exists().

The following patch will implement reading the reverse index data from
the MIDX itself.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Reviewed-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>