[skara] RFR: Allow tags to be found in multiple branches

[Robin Westberg]

Repositories that configure different fixVersions for different branches should allow a tag to be found in more than one of them, just like it would work if they were different repositories.

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Commit messages:
 - Allow tags to be found in multiple branches, as they will end up updating different issues

Changes: https://git.openjdk.java.net/skara/pull/1088/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1088&range=00
  Stats: 170 lines in 2 files changed: 117 ins; 25 del; 28 mod
  Patch: https://git.openjdk.java.net/skara/pull/1088.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1088/head:pull/1088

PR: https://git.openjdk.java.net/skara/pull/1088