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Install Meld > Add it to SystemPath where Meld.exe is located.
 SourceTree > Tools > Options > Diff > Diff(Custom > Meld.ext) & Merge(Custom > Meld.exe)
 Arguements:
Diff: $LOCAL $REMOTE
Merge: $LOCAL $BASE $REMOTE --auto-merge --output=$MERGED
 If you got DLL Issue :
Copy "C:\Program Files (x86)\Meld\lib\libgirepository-1.0-1.dll" to "C:\Program Files (x86)\Meld\libgirepository-1.0-1.dll". 
One Directory Up.