RFR: 2106: GitHub secondary rate limit causing delays for /backport command

[Erik Joelsson]

We are hitting weird "secondary rate limit" problems when fetching branches for the fork repository in the BackportCommand. To try to alleviate this problem, I'm replacing that call to just fetch the single branch we actually want to find (instead of iterating through all to find it). This may not look like a big change, but the number of branches in some fork repos is now forcing 5+ pagination calls, so this single call should reduce the volume significantly. It's still unclear exactly why this particular call is triggering the rate limiter.

The actual problem happens around line 262, but I figured I could apply the same thing on line 187 as well.

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Commit messages:
 - SKARA-2106

Changes: https://git.openjdk.org/skara/pull/1587/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1587&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2106
  Stats: 5 lines in 1 file changed: 0 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/skara/pull/1587.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1587/head:pull/1587

PR: https://git.openjdk.org/skara/pull/1587