git-svn: avoid leaving leftover committer/author info in rebase

git-svn: avoid leaving leftover committer/author info in rebase

We set the 6 environment variables for controlling
committer/author email/name/time for every commit.

We do this in the parent process to be passed to
git-commit-tree, because open3() doesn't afford us the control
of doing it only in the child process.  This means we leave them
hanging around in the main process until the next revision comes
around and all 6 environment variables are overwridden again.

Unfortunately, for the last commit, leaving them hanging around
means the git-rebase invocation will pick it up, rewriting the
rebased commit with incorrect author information.  This should fix
it.

Signed-off-by: Eric Wong <normalperson@yhbt.net>