RFR: 584: additional check for "git sync"

[Erik Helin]

Hi all,

please review this patch that makes `git-sync` check if the `--from` and `--to` arguments refer to the same repository. If they do, then `git-sync` exits with an error.

Testing:
- [x] Manual testing on Linux x64

Thanks,
Erik

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Commit messages:
 - 584

Changes: https://git.openjdk.java.net/skara/pull/906/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=906&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-584
  Stats: 14 lines in 2 files changed: 14 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/skara/pull/906.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/906/head:pull/906

PR: https://git.openjdk.java.net/skara/pull/906