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This CL adds a new mode to print_histogram_names.py to print the added
and removed histograms relative to a git revision e.g.:

./print_histogram_names.py --diff origin/master
./print_histogram_names.py --diff HEAD~

This is especially useful for CLs which add lots of histogram suffixes,
so that it is easy to learn precisely what histograms are generated by
the change.

Documentation is also updated to encourage using this tool when using
histogram suffixes.

Bug: None
Change-Id: Ibc1a8cde4216ad49f19a795bbfaa31dc61451705
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2205330
Commit-Queue: Charlie Harrison <csharrison@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#770364}