RFR: git-pr-fetch: remove --branch option

[Erik Helin]

Hi all,

please review this patch that removes the `--branch` option from `git pr fetch` to make `git pr fetch` align better
with `git fetch`.

Thanks,
Erik

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Commit messages:
 - git-pr-fetch: remove --branch option

Changes: https://git.openjdk.java.net/skara/pull/793/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=793&range=00
  Stats: 11 lines in 1 file changed: 0 ins; 10 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/793.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/793/head:pull/793

PR: https://git.openjdk.java.net/skara/pull/793