Revert "Deploy git 2.18 to depot_tools on a bleeding edge."

This reverts commit d61997d3.

Reason for revert: known to not work on mac, at least.

Original change's description:
> Deploy git 2.18 to depot_tools on a bleeding edge.
> 
> R=​hinoka
> 
> Bug: 851505
> Change-Id: I3e13da0ec736fb3da4f4a95c71ed07c9a4fd0eca
> Reviewed-on: https://chromium-review.googlesource.com/1168130
> Reviewed-by: Ryan Tseng <hinoka@chromium.org>
> Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>

TBR=hinoka@chromium.org,tandrii@chromium.org

Change-Id: Ie4c12e67cd50a9a4610a69dd55c43f444d79a9dd
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 851505
Reviewed-on: https://chromium-review.googlesource.com/1168350Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>

bootstrap/win/manifest_bleeding_edge.txt

@@ -17,4 +17,4 @@ $VerifiedPlatform windows-386 windows-amd64
 infra/python/cpython/${platform} version:2.7.13.chromium7
 
 @Subdir git
-infra/git/windows-${arch} version:2.18.0.chromium16
+infra/git/windows-${arch} version:2.17.1.chromium15