The commit-graph file is a very helpful feature for speeding up git
operations. In order to make it more useful, make it possible to
write the commit-graph file during standard garbage collection
operations.

Add a 'gc.commitGraph' config setting that triggers writing a
commit-graph file after any non-trivial 'git gc' command. Defaults to
false while the commit-graph feature matures. We specifically do not
want to have this on by default until the commit-graph feature is fully
integrated with history-modifying features like shallow clones.

So I played around with an earlier version of this patch series a
while ago... and as I looked into my gitconfig today I was surprised
to have both 'core.commitGraph' and 'gc.commitGraph' config variables
set.  When I looked into it I came across this email from Ævar:

  https://public-inbox.org/git/87fu3peni2.fsf@evledraar.gmail.com/

  > Other than the question if 'gc.commitGraph' and 'core.commitGraph'
  > should be independent config variables, and the exact wording of the
  > git-gc docs, it looks good to me.

  Sans doc errors you pointed out in other places (you need to set
  core.commitGraph so it's read at all), I think it's very useful to have
  these split up. It's simliar to pack.useBitmaps & pack.writeBitmaps.

I think the comparison with pack bitmaps makes a lot of sense and I
have to say that I really like those 'useBitmaps' and 'writeBitmaps'
variable names, because it's clear right away which one is which,
without consulting the documentation.  I think having 'useCommitGraph'
and 'writeCommitGraph' variables would be a lot better than the same
variable name in two different sections, and I'm sure that then I
wouldn't have been caught off guard.

Yeah, I know, my timing sucks, with 'core.commitGraph' already out
there in the -rc releases...  sorry.