filter-branch: use git-sh-setup's ident parsing functions

filter-branch: use git-sh-setup's ident parsing functions

This saves us some code, but it also reduces the number of
processes we start for each filtered commit. Since we can
parse both author and committer in the same sed invocation,
we save one process. And since the new interface avoids tr,
we save 4 processes.

It also avoids using "tr", which has had some odd
portability problems reported with from Solaris's xpg6
version.

We also tweak one of the tests in t7003 to double-check that
we are properly exporting the variables (because test-lib.sh
exports GIT_AUTHOR_NAME, it will be automatically exported
in subprograms. We override this to make sure that
filter-branch handles it properly itself).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>