I am asking here to reopen this issue https://jira.atlassian.com/browse/SRCTREEWIN-14248 because who has examined and closed it without waiting for my feedback.clearly didn't understand what I was talking about.

SourceTree insists in staging unmodified files (still containing merge conflicts) when I press the Abort button THAT IS PART OF SOURCETREE USER INTERFACE. the external tool is irrelevant and its exit code is irrelevant. (I am referring to sourcetree's dialog that stays open while the merge tool is running which contains a misleading "abort" button that does nothing)

Moreover: even if the issue was the external tool giving a wrong exit code, It would be a huge user experience improvement if, before staging files after having launched the merge tool you checked for the presence of merge conflict markers in the files.

  Even the 20 years old WinCVS warns you with "the file you are trying to commit appears to  be still containing contain merge conflict markers, are you sure you want to proceed"?