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>