Fix permalinks to items in Perforce depots

[peterguy]

p4-fusion calculates Git commit hashes while syncing depots, but if certain things in the depot change and the depot is re-synced, (known issue: Perforce users being deactivated, thus losing access to the email address), the commit hashes could change, destroying permalinks because permalinks use commit hashes. Either address the instability of commit hashes generated by p4-fusion, or switch to using changelist and/or file revisions in permalinks for Perforce depots.

[peterguy]

After #40330, this issue will add permalink support.