rebase: update invocation of rebase dot-sourced scripts

rebase: update invocation of rebase dot-sourced scripts

Due to historical reasons, the backend scriptlets for "git rebase"
are structured a bit unusually. As originally designed,
dot-sourcing them from "git rebase" was sufficient to invoke the
specific backend.

However, it was later discovered that some shell implementations
(e.g. FreeBSD 9.x) misbehaved by continuing to execute statements
following a top-level "return" rather than returning control to
the next statement in "git rebase" after dot-sourcing the
scriptlet. To work around this shortcoming, the whole body of
git-rebase--$backend.sh was made into a shell function
git_rebase__$backend, and then the very last line of the scriptlet
called that function.

A more normal architecture is for a dot-sourced scriptlet merely
to define functions (thus acting as a function library), and for
those functions to be called by the script doing the dot-sourcing.
Migrate to this arrangement by moving the git_rebase__$backend
call from the end of a scriptlet into "git rebase" itself.

While at it, remove the large comment block from each scriptlet
explaining this historic anomaly since it serves no purpose under
the new normalized architecture in which a scriptlet is merely a
function library.

Signed-off-by: Wink Saville <wink@saville.com>
Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>