RFR: 1320: Skara failing to create backport records for fixes synced into mainline

[Erik Joelsson]

When forward merging changes from jdk18 to mainline, we are no longer correctly creating backport entries for jdk19. This is because we switched from a Skara configuration for fixVersion to using the .jcheck/conf version from the repo in mainline. When Skara figures out the correct fixVersion from .jcheck/conf, it uses the file at the commit in question. In the case of a forward merge, that commit will have an outdated version in .jcheck/conf, as the commit is not a descendant of the version update change.

This change fixes this situation, by optionally (through a configuration parameter) have Skara use the HEAD version of .jcheck/conf (for the given branch) when resolving fixVersion for a commit. Some care needs to be taken when using this option, but essentially no more so than we already had to do when configuring the fixVersion in the Skara config.

The plan is to enable this option just for mainline jdk and jfx.

-------------

Commit messages:
 - SKARA-1320

Changes: https://git.openjdk.java.net/skara/pull/1276/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1276&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-1320
  Stats: 82 lines in 4 files changed: 79 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/skara/pull/1276.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1276/head:pull/1276

PR: https://git.openjdk.java.net/skara/pull/1276