RFR: cli: add trees command

[Erik Helin]

Hi all,

please review this patch that adds
[trees](https://hg.openjdk.java.net/code-tools/trees) functionality to Skara.
This is mainly for those who want to use Skara for working with Mercurial today
and are missing the trees functionality, but I also added a couple of trees
"commands" like `tstatus`. `tpull` etc. to `skara.gitconfig`.

Testing:
- Manual testing of both Mercurial and Git trees commands

Thanks,
Erik

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Commit messages:
 - cli: add trees command

Changes: https://git.openjdk.java.net/skara/pull/599/files
 Webrev: https://webrevs.openjdk.java.net/skara/599/webrev.00
  Stats: 222 lines in 5 files changed: 210 ins; 0 del; 12 mod
  Patch: https://git.openjdk.java.net/skara/pull/599.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/599/head:pull/599

PR: https://git.openjdk.java.net/skara/pull/599