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This adds commit history to named commits in the CommitTracker. The history can be viewed by requesting /_query_commit/<commit-name>.

This also adds a _reset_commit servlet which will reset the current commit ID of a named commit. This servlet will only obey the request if the given commit ID is present in the named commit's history.

BUG=None
R=kalman@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/736773002

Cr-Commit-Position: refs/heads/master@{#314395}