Revert "override rather than setting user.name and user.email"

Revert "override rather than setting user.name and user.email"

This reverts commit 66eb63dd820ade940b26b8eb00759d1b0f9242a0.

git-annex init is the only thing that uses ensureCommit. So overriding
there will make later commits to the git-annex branch or by git-annex sync
fail.

It's ugly that git-annex init sets user.name and user.email, but it only
does it on systems that are badly configured.