t6600-test-reach: generalize *_three_modes

t6600-test-reach: generalize *_three_modes

In a preparatory step to implement generation number v2, we add tests to
ensure Git can read and parse commit-graph files without Generation Data
chunk. These files represent commit-graph files written by Old Git and
are neccesary for backward compatability.

We extend run_three_modes() and test_three_modes() to *_all_modes() with
the fourth mode being "commit-graph without generation data chunk".

Signed-off-by: Abhishek Kumar <redacted>
Reviewed-by: Taylor Blau <redacted>
Reviewed-by: Derrick Stolee <redacted>
Signed-off-by: Junio C Hamano <redacted>