RFR: cli: add --ignore flag to git-sync

[Erik Helin]

Hi all,

please review this patch that adds the `--ignore` flag to `git-sync`. The `--ignore` flag is a regular expression for branches that should *not* be synced. The default value for `--ignore` is `pr/.*` to avoid sync branches created by Skara to facilitate pull requests with dependencies.

Testing:
- [x] Manual testing on Linux x64

Thanks,
Erik

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Commit messages:
 - cli: add --ignore flag to git-sync

Changes: https://git.openjdk.java.net/skara/pull/907/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=907&range=00
  Stats: 22 lines in 1 file changed: 22 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/skara/pull/907.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/907/head:pull/907

PR: https://git.openjdk.java.net/skara/pull/907