Provide xtask command to build combined Gource file

[IsaacWoods]

Visualising the whole project's history in Gource would be cool, which would involve merging multiple repositories into one tree. This is luckily quite easy in Gource; we just have to construct and sort logs for each repo, and then rename them. An example with the main repo + rust-osdev/acpi:

gource --output-custom-log gource_acpi.txt lib/acpi -s 0.1 -s 0.1               # Not sure if passing `-s 0.1 -a 0.1` does anything here
sed -i -r "s#(.+)\|#\1|/lib/acpi#" gource_acpi.txt                              # This renames the paths in the sub-repo
gource --output-custom-log gource_main.txt . -s 0.1 -a 0.1
cat gource_acpi.txt gource_main.txt | sort -n >gource.txt                       # Combine and sort all the entries
gource -a 0.1 -s 0.1 gource.txt

It would be cool to have an xtask command that could produce a Gource log that contained all of the repos that we consider part of the 'main' project, for some definition of that.

[IsaacWoods]

I've also just found out you can add captions to Gource very easily. We could have significant events (e.g. renamed to Poplar, support for riscv started, etc.) in Rust and then export them with unix timestamps for ease