Extra pile checks for format-patch

[lucasdemarchi]

One common mistake is to start preparing a patch series and before finishing it do a git fetch. When this is done the RESULT_BRANCH in the remote may not match the one the feature/fix started from. When we do a git pile format-patch it implicitly uses the remote branch to compare with git range-diff. That range-diff will not make sense if it contains changes outside what we really changed.

One idea to cope with that is: assuming pile_branch and result_branch from the remote are always in sync (i.e. nobody is pushing result_branch but rather generating from it), check if the merge-base of pile and origin/pile is origin/pile. If it is something else, it means the branches diverged and we should instruct the user to use the longer form of format-patch, passing a sensible result_branch for comparison.