[phobos] Integration branches

[Martin Nowak]

With the recent inflation of merging pull requests and reverting them I  
wonder if it would
make sense to use integration branches.

The proposed workflow would be.

  - merge pull request to integration branch of dmd/druntime/phobos, i.e.  
multiple merges
    are stashed into the same branch
  - if auto testing or something else fails revert merge and discuss follow  
ups on github
  - after everything is sorted out merge the complete pull request into  
master
  - from time to time the integration branch needs to be synced with master
    most likely when not too much pulls are merged into it

That way pull requests can stay open until being functional and merged to  
master.
The maintenance overhead should be fairly low.
The auto tester would need to test two sources and sometimes one needs to  
reset the integration branch.

martin