git: Add Support for Clone No-op for Commit pinned clones

[pjbgf]

Commit pinned clones do not support the use of LastRevision to shortcirtuit the clone operation.
This means, specially for libgit2, that full clones are executed, causing higher resource consumption.

This sparkled as part of a cncf thread in which a user reported source controller hitting 6000% CPU saturation and 3600% memory saturation with the default resources requests/limits. That resource consumption was observed with 57 sources (most are pinned to a specific git hash).

The implementation could follow similar logic as checkout for branches:
https://github.com/fluxcd/source-controller/blob/3013b5fc3df274d790d017ef98596f3e95073dd9/pkg/git/libgit2/checkout.go#L103-L120