merge-tree.c: add --merge-base=<commit> option

merge-tree.c: add --merge-base=<commit> option

This patch will give our callers more flexibility to use `git merge-tree`,
such as:

    git merge-tree --write-tree --merge-base=branch^ HEAD branch

This does a merge of HEAD and branch, but uses branch^ as the merge-base.

And the reason why using an option flag instead of a positional argument
is to allow additional commits passed to merge-tree to be handled via an
octopus merge in the future.

Signed-off-by: Kyle Zhao <kylezhao@tencent.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>