Investigate `gitserver` changes to support Perforce depots

[peterguy]

Subsequent to #50423, we need to figure out how to change gitserver so that it can read the Perforce depot clone to get all of the information it's expecting.

[peterguy]

This is where we will start to run into issues with the lack of commit hashes and other git features.

See the docs page enumerating gitserver's API and the internal spreadsheet discussing Perforce equivalents

[peterguy]

The outcome of this issue should be documentation, comments, guidelines, and knowledge enough to add an effort estimate label to #50420