test-reach: test is_descendant_of

test-reach: test is_descendant_of

The is_descendant_of method takes a single commit as its first parameter
and a list of commits as its second parameter. Extend the input of the
'test-tool reach' command to take multiple lines of the form
"X:<committish>" to construct a list of commits. Pass these to
is_descendant_of and create tests that check each result.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>