Update depot_tools prod win git to 2.15.1.2

R=iannucci

Bug: 749709
Change-Id: I2b2dbde5ac74e20c36da0c691999f9dc09ea7ca5
Reviewed-on: https://chromium-review.googlesource.com/806996Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Aaron Gable <agable@chromium.org>

bootstrap/win/manifest.txt

@@ -17,4 +17,4 @@ $VerifiedPlatform windows-386 windows-amd64
 infra/python/cpython/windows-386 version:2.7.6
 
 @Subdir git
-infra/git/${platform} version:2.15.0.chromium12
+infra/git/windows-${arch} version:2.15.1.2.chromium12