Cannot abort external conflict merges - file is always marked as resolved

[Ben Capewell]

I'm using SourceTree for Windows v2.1.2.3.

I'm unable to abort merges launched in an external merge tool - even after I've clicked abort in the modal dialogue opened by SourceTree, when I close the external merge tool (without saving the tmp file), SourceTree marks the conflicted file as resolved.

I'm using WinMerge as a custom merge tool, with the command "C:\Program Files (x86)\WinMerge\WinMergeU.exe" and argument "$MERGED" to make use of WinMerge's conflict file merge facility.