commit-graph idea: warn when disabled for incompatible repositories

[Thomas Ferris Nicolaisen <tfnico@gmail.com>]

Hey there,

I recently started dabbling with commit-graph.

Accidentally, my first use-case was a local copy of a big repository
(chromium source) that another developer had cloned shallow (I wasn't
even aware of their clone being shallow at that point).

After spending a little time trying to figure out why no commit-graph
file was being created, I noticed that it worked just fine testing in
a fresh git repo.

Then I discovered the .git/shallow file in the big repo. So I did
fetch --unshallow, and commit-graph started working. Taking a 20
second log --graph operation down to less than a second (wooo!).

I saw some recent release notes that mentions that commit-graph is
disabled in incompatible repositories (graft, replace). I assume this
also be the case for shallow clones.

Here's the idea that may help others on the same path: Some warning
output when attempting to use commit-graph when it is disabled (either
by configuration or automatically).

I think others that come across commit-graph may have tried such
tricks (like shallow clones) to better work with their repositories,
and it could be frustrating that commit-graph has no apparent effect.

Apologies if this is a stupid/bad/old idea!