feat: add team city support

[tagoro9]

This PR adds support for Team City.

Team City has all the build related information in certain configuration parameters, but in order to make them available to greenkeeper-lockfile, they need to be manually mapped to environment variables in the build configuration.

This PR assumes that these two variables are available to the script:

• VCS_ROOT_URL which is mapped to the vcsroot.<vcsrootid>.url parameter
• VCS_ROOT_BRANCH which is mapped to the teamcity.build.branch parameter

The GH_TOKEN environment variable also needs to be defined, but that needs to happen independently of the CI server.

In order to detect if builds are coming from a PR or not, we can check the name of the branch. If it ends with /merge or /head, it means it is a pull request according to this post.

In order to set the firstPush and correctBuild flags correctly I copied the logic from the Codeship implementation. Unlike Codeship, Team City does not provide any parameter to access the commit message, so I added a function in gitHelpers to do that.

Edit: I just realized that #106 is also trying to add support to Team City. I hadn't seen it until I created the PR. There are some differences between both PRs. I'm happy to close this PR if the other path is preferred. Sorry for the noise.

[janl]

This looks great, thanks a lot!

[greenkeeperio-bot]

🎉 This PR is included in version 1.15.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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