Add support for Mercurial repository

[enricomariam42]

With Mercurial bridge from Git project seems straightforward.
From: https://github.com/mnauw/git-remote-hg
"allows you to clone, fetch and push to and from Mercurial repositories as if they were Git ones"
"add the 'git-remote-hg' script anywhere in your $PATH (with Mercurial installed)"
But I can't get it to work.

[HiromuHota]

pdi-git-plugin uses EGit, Java implementation of Git client, and does not use Git CLI.
git-remote-hg looks to me that it assumes the use of Git CLI, meaning it cannot be used in pdi-git-plugin (and that's why you couldn't get it working).
In order to support Mercurial, I think pdi-git-plugin has to use a Java implementation of Mercurial client such as Hg4J.

Currently, org.pentaho.di.git.spoon.model.IVCS is the common interface, implemented by org.pentaho.di.git.spoon.model.UIGit (for Git) and by org.pentaho.di.git.spoon.model.SVN for (Subversion).
Similarly, a support for Mercurial is possible by implementing the interface org.pentaho.di.git.spoon.model.IVCS and uses Hg4J internally.

If anyone can implement, please send me a PR. I'm happy to review and merge it.

[HiromuHota]

Small correction
Wrong: pdi-git-plugin uses EGit
Correct: pdi-git-plugin uses JGit