Resolve merge conflicts with 3-way merge

[tiran]

ipatool doesn't allow 3-way merge to resolve merge conflicts:

$ ./ipatool pr-push 1363 -r abok
Reviewer: Alexander Bokovoy <abokovoy@redhat.com>
Will apply 1 patches to: master
Fetching...
Aplying to master: Replace nose with unittest and pytest
git am
Applying: Replace nose with unittest and pytest
Patch failed at 0001 Replace nose with unittest and pytest
The copy of the patch that failed is found in: .git/rebase-apply/patch
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".
error: patch failed: ipatests/test_webui/ui_driver.py:220
error: ipatests/test_webui/ui_driver.py: patch does not apply
→ 128
Command failed

The same merge works fine with rebase:

$ git rebase upstream/master 
First, rewinding head to replay your work on top of it...
Applying: Replace nose with unittest and pytest
Using index info to reconstruct a base tree...
M       ipatests/test_webui/ui_driver.py
Falling back to patching base and 3-way merge...
Auto-merging ipatests/test_webui/ui_driver.py

PR freeipa/freeipa#1363

[frasertweedale]

@tiran do you still want this feature in ipatool itself, or leave it as something to be handled separately by the PR author?