add time parsing for before/after fields

Created by: camdencheek

This commit adds time parsing for before/after fields that attempts to replicate the parsing git does for its time arguments. This allows us to

1. validate that the time is valid before attempting to run a git command, and
2. use native go time types for the new commit/diff search path

It uses go-naturaldate to replicate the more human-like date formats git accepts like "2 hours ago".

Fixes #25176 (closed)