RFR: forge: add search method

[Erik Helin]

Hi all,

please review this patch that adds the `Forge.search` method to allow searching for a commit with a particular hash on
a forge.

Thanks,
Erik

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Commit messages:
 - forge: add search method

Changes: https://git.openjdk.java.net/skara/pull/854/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=854&range=00
  Stats: 290 lines in 13 files changed: 248 ins; 25 del; 17 mod
  Patch: https://git.openjdk.java.net/skara/pull/854.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/854/head:pull/854

PR: https://git.openjdk.java.net/skara/pull/854