Revert "rebase: fix run_specific_rebase's use of "return" on FreeBSD"

Revert "rebase: fix run_specific_rebase's use of "return" on FreeBSD"

This reverts commit 99855ddf4bd319cd06a0524e755ab1c1b7d39f3b.

The workaround 99855ddf introduced to deal with problematic
"return" statements in scripts run by "dot" commands located
inside functions only handles one part of the problem.  The
issue has now been addressed by not using "return" statements
in this way in the git-rebase--*.sh scripts.

This workaround is therefore no longer necessary, so clean
up the code by reverting it.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>
Acked-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>