feature request: allow commit.email config setting

As part of my dayjob, I did and still do some work on an upstream
project. A while ago, I was granted commit access to that project.
However, upstream asked that I would register with their system using a
private email, or at least one that wouldn't change if I changed jobs,
rather than my work email.

Now, I (and my employer) would like that the work I do as part of my
current job on that project has my work email in the Author field, but
since the commit access was granted to me privately/personally, it would
be nice if the Committer email was the one I used to register with their
system. I can set GIT_COMMITTER_EMAIL in the environment, but that is
rather inconvenient, since that means I have to remember to do that in
the shell I'm using for that particular project, and I can't use that
shell for other projects. So it would be really nice if I could set
commit.email = $private-email in the local .git/config for that
particular project. I don't personally have a use for commit.name (when
missing, that should just use user.name as usual), but it would probably
be most consistent to allow that too.

I tried looking into ident.c, but it doesn't seem like it is
straight-forward to implement. Probably fmt_ident, ident_default_email
etc. would need to be passed information about what purpose the ident is
to be used for. So before trying to implement this, I want to hear if
this is a reasonable thing to support. Also, I'm sure there are some
subtle semantics that would need to be decided and gotchas to watch out for.

Rasmus