localrepo: Use catfile cache in ReadObject

A localrepo already has access to the catfile cache, and ReadCommit() already uses it to read objects. ReadObject() still was spawning a git-cat-file(1) process itself. This change modifies ReadObject() to use the catfile cache. This might avoid actually spawning a new process. We verify this with the newly introduced gittest.CountingCommandFactory, and benchmark the effect on FindLicense.

The change is behind a feature flag rolled out through: #4662 (closed)