Rebase a Pull Request

When working on PR it is possible for other changes to get merged into

the parent branch that conflict with your branch. The conflict can be

trivial, for example both the parent branch and you branch add an

entry to the top of `CHANGELOG`. Git can not unambiguously tell what

to with both changes (should one go above the other? if so, which

order? should it try to merge them?) so it gives up and declare the

branches can not be merged cleanly. If you were the branches at the

command line you could do an interactive merge where git pauses half

way through to give you a chance to resolve the conflicts by hand,

however using github almost all of the merges back into the parent

branches are done via the web-interface, so only PRs which will

cleanly merged will be accepted. If this happens to your PR, one of

the developers will ask you to 'rebase' your branch which is the

process by which you resolve the conflicts between your branch and

the parent branch.

In git rebasing is a mild form of re-writing history, as it

effectively transplants where your branch from where you intially

forked of off the parent branch to some other point. For a much more

detailed explanation (with pictures!) see

http://git-scm.com/book/en/Git-Branching-Rebasing. In general,

re-writing history (particularly published history) is considered very

bad, but in this case is very useful.