SourceTree hangs when launching external diff (FileMerge)

When doing an external diff with FileMerge (may happen with other apps as well), SourceTree will hang if FileMerge isn't already running. This is 100% reproducible:

1. In Preferences, select FileMerge as the visual diff tool.
2. Make sure FileMerge is not already running.
3. Do an external diff, which will launch FileMerge.

Result: SourceTree will hang (beachball) until you quit FileMerge – closing the file window will not suffice.

Expected result: SourceTree should continue to respond and operate independently of FileMerge.

Workaround: In Xcode, launch FileMerge manually (Xcode->Open Developer Tool->FileMerge) before doing an external diff. If it's already running, the hang won't occur.

Suspected cause: SourceTree launches the external diff application synchronously and must therefore wait until it exits. It should be launched asynchronously.