Replace calls to git.ExecReader with specific use cases

Created by: ryanslade

Instead of calling directly into git.ExecReader, we should instead call into functions that show the intent.

For example: https://github.com/sourcegraph/sourcegraph/blob/main/enterprise/cmd/frontend/internal/codeintel/resolvers/position.go#L126

We should instead be calling a function named something like git.ReadDiffHunks.

Once all calls are changed, remove git.ExecReader