From: Nikolaus Schuetz <nikolauspschuetz@gmail.com>

`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 <nikolauspschuetz@gmail.com>

Submitted-As: https://lore.kernel.org/git/pull.2186.git.1784998828879.gitgitgadget@gmail.com