RFR: Include 'treconfigure' in help for git trees

[Magnus Ihse Bursie]

This fix makes "git trees --help" a bit more helpful.  It also add missing newline separating sub-repos in output.

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Commit messages:
 - Include 'treconfigure' in help for git trees

Changes: https://git.openjdk.java.net/skara/pull/786/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=786&range=00
  Stats: 4 lines in 1 file changed: 3 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/786.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/786/head:pull/786

PR: https://git.openjdk.java.net/skara/pull/786