merge-base: add tests for --is-ancestor

merge-base: add tests for --is-ancestor

`git merge-base --is-ancestor A B` is used a lot in scripts but has no
tests. Add some to t6010 covering its exit codes: 0 when A is an
ancestor of B, 1 when it is not, and 128 (not 1) when given a bad
argument. Also check that --is-ancestor and --all can't be combined,
and that the resulting error names both options.

Signed-off-by: Nikolaus Schuetz <redacted>
Signed-off-by: Junio C Hamano <redacted>