RFR: trees: add treconfigure command

[Erik Helin]

Hi all,

please review this patch that adds the `hg/git treconfigure` command. The
`treconfigure` can be used to re-initialize the trees configuration for a set of
repositories. This is useful when a user has, either by mistake or on purpose,
renamed and/or moved a repository in a "forest".

_Note_: the existing `trees.py` extension offered the command `hg tconfig`, but
that command clashes with the `config` command present in both `hg` and `git` (a
user can't run the command `config` in all repositories in a "forest"). For the
Skara port of `trees.py` I opted to instead provide the `treconfigure` command
to avoid name clashes.

Testing:
- Manual testing of `git treconfigure` and `hg treconfigure`

Thanks,
Erik

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Commit messages:
 - trees: add treconfigure command

Changes: https://git.openjdk.java.net/skara/pull/612/files
 Webrev: https://webrevs.openjdk.java.net/skara/612/webrev.00
  Stats: 60 lines in 3 files changed: 56 ins; 1 del; 3 mod
  Patch: https://git.openjdk.java.net/skara/pull/612.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/612/head:pull/612

PR: https://git.openjdk.java.net/skara/pull/612