Add support for sourcing chart from head ref when testing pull request

[mksh]

This is based on https://github.com/stefanprodan/hrval-action/pull/33 and intended to solve the same problem.

How to clone a right helm chart source in the case when helm chart referenced in a helm release being tested by hrval-action is located in the same git repository, where hrval-action is run ?

In case of pull request, if a pull request base repository branch is referenced in helm release, hrval-action might be executed against invalid version of helm chart source, other than one contained in head repository branch.

Algorithm:

• when HRVAL_BASE_BRANCH and HRVAL_HEAD_BRANCH environment variables are set, for each helm chart being tested,
  • detect if Helm Release chart reference contains link to a repository where hrval-action currently runs and the ref specified equals to value specified in HRVAL_BASE_BRANCH environment variable
  • if such Helm Release have been encountered, fetch chart source from git ref specified in HRVAL_HEAD_BRANCH environment variable
  • otherwise, proceed with original ref setting

This patch is non-intrusive, and a pre-patch behavior will be executed in case if HRVAL_BASE_BRANCH and HRVAL_HEAD_BRANCH variables are not set.

[mksh]

Sure, have added documentation for this feature to Readme