RFR: git-defpath: add support for --upstream and --fork

[Erik Helin]

Hi all,

please review this patch that adds support for the `--upstream` and `--fork` arguments to `git-defpath` (note that `git defpath --fork` might fork the repo as a side-effect). I also made the rest of the arguments to `git-defpath` configurable.

Thanks,
Erik

## Testing
- [x] Manual testing of `git-defpath`

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Commits:
 - 5dc9584c: git-defpath: add support for --upstream and --fork

Changes: https://git.openjdk.java.net/skara/pull/302/files
 Webrev: https://webrevs.openjdk.java.net/skara/302/webrev.00
  Stats: 131 lines in 1 file changed: 115 ins; 7 del; 9 mod
  Patch: https://git.openjdk.java.net/skara/pull/302.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/302/head:pull/302

PR: https://git.openjdk.java.net/skara/pull/302