GitLab

This will allow clients that use the subtree mirror to specify a
revision for jsoncpp without needing nested DEPS.

Also cleans up the gitignore for the subtree mirror so gclient
revert will work properly.

Bug: 1017514
Change-Id: I3832b7fadc0fa9ddcc5dad700690b09e374a4f67
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1877455Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709541}