Add the ability to forwardport a change

[dakrone]

Occasionally I do work on a non-master branch and want to forwardport a PR to the master branch, however, even if I manually specify the SHA and branch:

~/es/elasticsearch (git) 0f312c8b  7.x [origin/7.x] (2 stashed)
» backport --sha 0f312c8b381fe19b69964f1ef677c9c46ed844b7 --branch master       
✖ Loading commit "0f312c8b"
No commit found on master with sha "0f312c8b381fe19b69964f1ef677c9c46ed844b7"

It appears to look for the sha on the master branch instead of 7.x.

It'd be great to be able to forwardport though!

[sorenlouv]

Hey!

This is already possible - but not very well documented, and it should probably work as you suggest.

By using the sourceBranch argument, you can pick commits from a non-master branch:

The following will list commits on 7.x and backport the selected commit to master:

backport --sourceBranch 7.x --branch master

The following currently doesn't work, but I'll look into fixing it:

backport --sourceBranch 7.x --branch master --sha 0f312c8b381fe19b69964f1ef677c9c46ed844b7

backport --branch master --sha 0f312c8b381fe19b69964f1ef677c9c46ed844b7