test-bot: fix start_revision for Travis

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.

Closes Homebrew/homebrew#44379.

Signed-off-by: Xu Cheng <xucheng@me.com>

0 files changed, 0 insertions, 0 deletions