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>

Library/Homebrew/cmd/test-bot.rb

@@ -318,6 +318,7 @@ module Homebrew
       # Use Travis CI Git variables for master or branch jobs.
       elsif ENV["TRAVIS_COMMIT_RANGE"]
         diff_start_sha1, diff_end_sha1 = ENV["TRAVIS_COMMIT_RANGE"].split "..."
+        diff_start_sha1 = git("merge-base", diff_start_sha1, diff_end_sha1).strip
       # Otherwise just use the current SHA-1 (which may be overriden later)
       else
         diff_end_sha1 = diff_start_sha1 = current_sha1