RFR: forge: add webUrl(Tag tag) method

[Erik Helin]

Hi all,

please review this small patch that adds support to `HostedRepository` for getting the web URL for a given tag.

Thanks,
Erik

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Commit messages:
 - forge: add webUrl(Tag tag) method

Changes: https://git.openjdk.java.net/skara/pull/1054/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1054&range=00
  Stats: 23 lines in 5 files changed: 23 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/skara/pull/1054.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1054/head:pull/1054

PR: https://git.openjdk.java.net/skara/pull/1054