RFR: make Backport <sha> case insensitive

[Ludvig Janiuk]

There's no reason why this should validate case, and has caused unnecessary friction in the past.

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Commit messages:
 - make Backport <sha> case insensitive

Changes: https://git.openjdk.org/skara/pull/1611/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1611&range=00
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/skara/pull/1611.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1611/head:pull/1611

PR: https://git.openjdk.org/skara/pull/1611