MergetoolDiffExchange has undesired behavior

[MoezGholami]

The current implementation of MergetoolDiffExchange for example, does not work for a situation where both sides of the working window are other valid windows.
For example, when one has BMR layout and is in the BASE window can normally put the diff in the MERGE window by MergetoolDiffExchangeRight. However, in the MERGE window, the diff from the BASE windo (left side) is not got with MergetoolDiffExchangeLeft, instead MergetoolDiffExchangeRight does the job (which is the opposite of what we intend).

Tracking the issue, I realized the DiffExchange function here is problematic. On MergetoolDiffExchangeLeft in MERGE window (BMR layout), it reverses the direction (which was a good thing in putting the diff from B to M with MergetoolDiffExchangeRight) and gets the diff from the REMOTE.
I believe we should not determine the direction by reversing it as it may consider another window which does not have to do anything with what we want (R in the BMR for example). We already know both exchange parties.
I have solved this issue locally and will send a pull request.

[MoezGholami]

#10 pull request resolves this issue.