Benchmark nedmalloc again, and re-enable it when appropriate

[dscho]

@kblees pointed out that MSys2-based Git for Windows is not compiled with nedmalloc enabled. That is most likely caused by an oversight of yours truly when porting MSys2's Git patches over to Git for Windows proper.

This is an excellent opportunity to revisit the nedmalloc performance.

[dscho]

Unfortunately, I re-enabled this (after light benchmarking which showed no noticable difference in performance). I say unfortunately because it made that Unicode bug resurface, requiring three hot fixes in rapid succession. My mistake: I should not have acted on the suggestion to re-enable nedmalloc without doing substantial (post release) research myself.