rebase: avoid non-function use of "return" on FreeBSD

rebase: avoid non-function use of "return" on FreeBSD

Since a1549e10, 15d4bf2e and 01a1e646 (first appearing in v1.8.4)
the git-rebase--*.sh scripts have used a "return" to stop execution
of the dot-sourced file and return to the "dot" command that
dot-sourced it.  The /bin/sh utility on FreeBSD however behaves
poorly under some circumstances when such a "return" is executed.

In particular, if the "dot" command is contained within a function,
then when a "return" is executed by the script it runs (that is not
itself inside a function), control will return from the function
that contains the "dot" command skipping any statements that might
follow the dot command inside that function.  Commit 99855ddf (first
appearing in v1.8.4.1) addresses this by making the "dot" command
the last line in the function.

Unfortunately the FreeBSD /bin/sh may also execute some statements
in the script run by the "dot" command that appear after the
troublesome "return".  The fix in 99855ddf does not address this
problem.

For example, if you have script1.sh with these contents:

run_script2() {
        . "$(dirname -- "$0")/script2.sh"
        _e=$?
        echo only this line should show
        [ $_e -eq 5 ] || echo expected status 5 got $_e
        return 3
}
run_script2
e=$?
[ $e -eq 3 ] || { echo expected status 3 got $e; exit 1; }

And script2.sh with these contents:

if [ 5 -gt 3 ]; then
        return 5
fi
case bad in *)
        echo always shows
esac
echo should not get here
! :

When running script1.sh (e.g. '/bin/sh script1.sh' or './script1.sh'
after making it executable), the expected output from a POSIX shell
is simply the single line:

only this line should show

However, when run using FreeBSD's /bin/sh, the following output
appears instead:

should not get here
expected status 3 got 1

Not only did the lines following the "dot" command in the run_script2
function in script1.sh get skipped, but additional lines in script2.sh
following the "return" got executed -- but not all of them (e.g. the
"echo always shows" line did not run).

These issues can be avoided by not using a top-level "return" in
script2.sh.  If script2.sh is changed to this:

main() {
        if [ 5 -gt 3 ]; then
                return 5
        fi
        case bad in *)
                echo always shows
        esac
        echo should not get here
        ! :
}
main

Then it behaves the same when using FreeBSD's /bin/sh as when using
other more POSIX compliant /bin/sh implementations.

We fix the git-rebase--*.sh scripts in a similar fashion by moving
the top-level code that contains "return" statements into its own
function and then calling that as the last line in the script.

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>