RFR: git-pr: do not assume a master branch

[Erik Helin]

Hi all,

please review this patch that makes `git pr` work even though no `master` branch
is present (which is the case if you clone a repository that uses a different
default branch). The fix consists of two parts:

- Use a better algorithm for detecting the target branch. `git pr create` will
  now pick the "closest" branch that exists in the upstream (parent) repository
  as the target branch by default.
- Read the .jcheck/conf file from the target branch

Testing:
- Manually tested `git pr create` on different repositories on Linux x64

Thanks,
Erik

-------------

Commits:
 - 1fa953ef: Merge branch 'master'
 - 447e865b: Read .jcheck/conf from target branch
 - 63a2a088: Fix a few bugs
 - 0d424208: git-pr: better algorithm for detecting target branch

Changes: https://git.openjdk.java.net/skara/pull/415/files
 Webrev: https://webrevs.openjdk.java.net/skara/415/webrev.00
  Stats: 78 lines in 4 files changed: 64 ins; 9 del; 5 mod
  Patch: https://git.openjdk.java.net/skara/pull/415.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/415/head:pull/415

PR: https://git.openjdk.java.net/skara/pull/415