Add ability to compare branches to bench_download tool

[Akirathan]

tools/performance/engine-benchmarks/bench_download.py tool, introduced in #5852, should provide functionality to compare two (or more?) branches. This will be useful when one wants to ensure that the changes on their PR won't cause any regressions after merging to develop.

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1. Introduce --branches <branch1, branch2, ...> option.
2. Display average for every branch under every graph, optimally with their differences.
3. Display points from all the branches in every graph. Branches will have different X axis (timestamp of benchmark runs).

[Akirathan]

I would like to implement this before #5998.

[enso-bot]

Pavel Marek reports a new STANDUP for today (2023-04-04):

Progress: Trying to add functionality to display more than one branch in the graphs, but gave that up after some struggling with Google charts and Jinja. I have at least created a PR where one can use --compare option that compares two benchmark runs from GH. It should be finished by 2023-04-06.