"Fixes #xxxx" now shows up in the issue log for non-default branches. #2190

For some time, I have been wondering why commits containing 'closing references' (like closes #xxxx) were not mentioned in the corresponding issues when pushed to a non-default branch.

So I tried to discover how it works -- hence learning Ruby! I don't expect that MR to pass, this is my very first attempt of contribution.

Update: my modifications are now done. To sum up:

• when a commit with a reference fixes #xxxx is pushed to a non-default branch, a cross-reference to that issue will be created;
• when that same commit is pushed to a default branch, no cross-reference will be created because a message This commit closes issue will be emitted.
• I also refined some of the existing tests and added 2 tests on the new behavior on non-default branches.