RFR: 596: Allow short-hand syntax for /label command

[Guoxiong Li]

Hi all,

This patch adds the short-hand syntax for `/label` command. The short-hand syntax makes the users easily add and remove the labels in one comment.

Some example:

Add a label: `/label +labelName`
Remove a label: `/label -labelName`
Mixed: `/label +labelName, -labelName2`
Mixed: `/label labelName, +labelName2, -labelName3`


And the test cases are added.

Thanks for taking the time to review.

Best Regards,
-- Guoxiong

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Commit messages:
 - 596: Allow short-hand syntax for /label command

Changes: https://git.openjdk.java.net/skara/pull/1255/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1255&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-596
  Stats: 223 lines in 2 files changed: 187 ins; 5 del; 31 mod
  Patch: https://git.openjdk.java.net/skara/pull/1255.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1255/head:pull/1255

PR: https://git.openjdk.java.net/skara/pull/1255