override rather than setting user.name and user.email

override rather than setting user.name and user.email

Avoid setting user.name and user.email in the git config when git is unable
to detect them.

git-annex has good reason to want to ensure git commit succeeds when eg
committing to the git-annex branch. But it's not playing nice to set these
values where other commands can see them.

Sponsored-by: Brett Eisenberg on Patreon