When a PR is merged during the test, the commit tree will be looked like
this:
master PR
\ /
\ /
|
root
The start_revision will be poined to the commit used to merging PR,
instead of common root commit. As result, `git diff-tree start end`
will list file changes both in PR and master branch, which later
contributes to additional formulae are tested in some Travis builds.
To fix the problem, we use `git merge-base` to get common ancestor as
the real start_revision.
ClosesHomebrew/homebrew#44379.
Signed-off-by: Xu Cheng <xucheng@me.com>
b37a285141