rebase: factor out sub command handling

rebase: factor out sub command handling

Factor out the common parts of the handling of the sub commands
'--continue', '--skip' and '--abort'. The '--abort' handling can
handled completely in git-rebase.sh.

After this refactoring, the calls to git-rebase--am.sh,
git-rebase--merge.sh and git-rebase--interactive.sh will be better
aligned. There will only be one call to interactive rebase that will
shortcut the very last part of git-rebase.sh.

Signed-off-by: Martin von Zweigbergk <martin.von.zweigbergk@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>