filter-branch: provide the convenience functions also for commit filters

filter-branch: provide the convenience functions also for commit filters

Move the convenience functions to the top of git-filter-branch.sh, and
return from the script when the environment variable SOURCE_FUNCTIONS is
set.

By sourcing git-filter-branch with that variable set automatically, all
commit filters may access the convenience functions like "map".

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>