Integrated: checkout: check status before converting

[Erik Helin]

Hi all,

please review this patch that makes the checkout bot checks if the `HEAD` commit on the branch in the `from` repository
already has been converted. If it has, then there is no need to run the converter.

Testing:
- [x] `make test` passes on Linux x64

Thanks,
Erik

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Commit messages:
 - checkout: check status before converting

Changes: https://git.openjdk.java.net/skara/pull/803/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=803&range=00
  Stats: 14 lines in 1 file changed: 10 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/skara/pull/803.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/803/head:pull/803

PR: https://git.openjdk.java.net/skara/pull/803