merge: small code readability improvement

merge: small code readability improvement

After our loop through the selected strategies, we compare best_strategy
to wt_strategy.  This is fine, but the fact that the code setting
best_strategy sets it to use_strategies[i]->name requires a little bit
of extra checking to determine that at the time of setting, that's the
same as wt_strategy.  Just setting best_strategy to wt_strategy makes it
a little easier to verify what the loop is doing, at least for this
reader.

Further, use_strategies[i]->name is used in a number of places, where we
could just use wt_strategy.  The latter takes less time for this reader
to parse (one variable name instead of three), so just use wt_strategy
to make the code slightly faster for human readers to parse.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>