Bitbucket Pipelines support

[glewiswp]

This PR introduces Bitbucket Pipelines support as discussed in #232.

Fixes #232.

[westonruter]

Shouldn't this be elif?

[westonruter]

Also, what is the +set for?

[glewiswp]

@westonruter The fi closes out the if statement on 78. This is essentially the fall back (when the environment variables TRAVIS and BITBUCKET_BRANCH are not set) which was on the original repo at 72-75.

As for +set, I was under the impression that this was needed to check if a variable was set in bash (I'm no bash master), instead of checking against a value. If it's incorrect I will gladly update.

[westonruter]

I think the structure should be:

	if [ "$TRAVIS" == true ]; then
		# ...
	elif [[ -n "${BITBUCKET_BRANCH+set}" ]]; then
		# ...
	else
		# ...
	fi

[westonruter]

The +set doesn't mean anything as far as I know. I think what you mean is:

[[ -n "${BITBUCKET_BRANCH}" ]]

Or equivalently:

[[ ! -z "${BITBUCKET_BRANCH}" ]]

As currently the -n "${BITBUCKET_BRANCH+set}" test will never be true, because "${BITBUCKET_BRANCH+set}" is interpolated as "+set" when the variable is empty, at least on Bash 3.2 on OSX.

Please make these changes and update the submodule in your PR to confirm they have the desired effect.

[glewiswp]

@westonruter Thanks for that explanation. That makes sense. The syntax above makes complete sense as well, I should have gone that route to begin with.

The statement [[ -n "${BITBUCKET_BRANCH+set}" ]] seemed to be working, but it could be on a newer version of Bash or could have adverse effects on Travis builds.

elif [[ ! -z "${BITBUCKET_BRANCH}" ]]; then works just as well in my testing.

Things are updated and tested and they look to be working well on my end.

[westonruter]

The [[ -n "${BITBUCKET_BRANCH+set}" ]] statement will seem to work because it will always work. By that I mean, it will always be true, since "+set" is not null.

[westonruter]

@glewiswp note that this will only compare with the previous commit which will lead to misleading results. See #197 and #229 for the same problem and a a proposed workaround for the same issue in Travis branch builds. What is needed here is the branch that the user intends to merge the changes into in order to be able to get the proper $DIFF_BASE. On Travis CI pull requests, this is available as $TRAVIS_BRANCH where it is defined as being the destination branch for where the changes will be merged. Nevertheless, for BitBucket it seems that the $BITBUCKET_BRANCH variable is not the pull request destination branch but rather the current branch, and thus it will have the exact same problem as Travis CI branch builds.