RFR: git-pr-approve: allow configurable message

[Erik Helin]

Hi all,

please review this patch that allows a configurable message for `git-pr approve` via the `-m, --message` flag.

Thanks,
Erik

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Commit messages:
 - git-pr-approve: allow configurable message

Changes: https://git.openjdk.java.net/skara/pull/789/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=789&range=00
  Stats: 13 lines in 2 files changed: 11 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/skara/pull/789.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/789/head:pull/789

PR: https://git.openjdk.java.net/skara/pull/789