merge: optimization to skip evaluate_result for single strategy

merge: optimization to skip evaluate_result for single strategy

For a merge with a single strategy, the result of evaluate_result() is
effectively not used and therefore is not needed, so avoid altogether.

On Windows, this optimization can halve the time required to perform a
recursive merge of a single commit with the LLVM repo.

Signed-off-by: Andrew Ng <redacted>
Signed-off-by: Junio C Hamano <redacted>