RFR: Initial version of CLI tool for hgupdate-sync label

[Robin Westberg]

An initial version of a hgupdate-sync-label CLI tool with associated refactorings, in preparation for further enhancements.

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Commit messages:
 - Initial version of CLI tool

Changes: https://git.openjdk.java.net/skara/pull/1002/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1002&range=00
  Stats: 279 lines in 16 files changed: 225 ins; 23 del; 31 mod
  Patch: https://git.openjdk.java.net/skara/pull/1002.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1002/head:pull/1002

PR: https://git.openjdk.java.net/skara/pull/1002