Update depot_tools bleeding-edge git to 2.15.0

Bug: 749709
Change-Id: I17beca5c374c080502833b30a26b68a0774689ef
Reviewed-on: https://chromium-review.googlesource.com/749633Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Aaron Gable <agable@chromium.org>

bootstrap/win/manifest_bleeding_edge.txt

@@ -15,4 +15,4 @@
 infra/python/cpython/${platform} version:2.7.13.chromium7
 
 @Subdir git
-infra/git/${platform} version:2.14.1.chromium10
+infra/git/${platform} version:2.15.0.chromium12