get_merge_base(): Change default base to `master...`

Add the range filter to try to remove commits that we don't care about.
Still not sure if this will work in all cases, as sometimes `..` seemed
to give me the right collection of files and sometimes `...` did.

code-review-difftool:
Pass the merge base without range dots (`..` or `...`) to Vim Fugitive,
as it doesn't accept revisions with ranges. Not sure what to do if I get
a range with two revision names yet, though.

Turns on Bash's `extglob` in order to use `?(...)` to include both
patterns in the expansion.

0 files changed, 0 insertions, 0 deletions