Fetch on submodule update

[Robert Dailey <rcdailey.lists@gmail.com>]

Problem: I want to avoid recursively fetching submodules when I run a
`fetch` command, and instead defer that operation to the next
`submodule update`. Essentially I want `fetch.recurseSubmodules` to be
`false`, and `get submodule update` to do exactly what it does with
the `--remote` option, but still use the SHA1 of the submodule instead
of updating to the tip of the specified branch in the git modules
config.

I hope that makes sense. The reason for this ask is to
improve/streamline workflow in parent repositories. There are cases
where I want to quickly fetch only the parent repository, even if a
submodule changes, to perform some changes that do not require the
submodule itself (yet). Then at a later time, do `submodule update`
and have it automatically fetch when the SHA1 it's updating to does
not exist (because the former fetch operation for the submodule was
skipped). For my case, it's very slow to wait on submodules to
recursively fetch when I only wanted to fetch the parent repo for the
specific task I plan to do.

Is this possible right now through some variation of configuration?