RFR: 1076: Not possible to use 'Backport <sha>' if original bug doesn't have public sha

[Erik Joelsson]

This patch adds a new way of declaring a pull requests to be a backport for situations where the original hash of the original change is not known or available. Technically such a PR should already work without the backport label, but it will display a warning about the issue not being open. By being able to declare a backport, that warning will be eliminated, which should make the user experience smoother and hopefully help avoid mistakes.

To use the new feature, you create a PR with a title "Backport <issueid>" instead of "Backport <hash>". Once this goes live, I will update the Skara wiki documentation.

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Depends on: https://git.openjdk.java.net/skara/pull/1185

Commit messages:
 - sha -> hash
 - SKARA-1076

Changes: https://git.openjdk.java.net/skara/pull/1186/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1186&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-1076
  Stats: 148 lines in 2 files changed: 143 ins; 1 del; 4 mod
  Patch: https://git.openjdk.java.net/skara/pull/1186.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1186/head:pull/1186

PR: https://git.openjdk.java.net/skara/pull/1186