rebase: refactor common shell functions into their own file

rebase: refactor common shell functions into their own file

The functions present in `git-legacy-rebase.sh` are used by the rebase
backends as they are implemented as shell script functions in the
`git-rebase--<backend>` files.

To make the `builtin/rebase.c` work, we have to provide support via
a Unix shell script snippet that uses these functions and so, we
want to use the rebase backends *directly* from the builtin rebase
without going through `git-legacy-rebase.sh`.

This commit extracts the functions to a separate file,
`git-rebase--common`, that will be read by `git-legacy-rebase.sh` and
by the shell script snippets which will be used extensively in the
following commits.

Signed-off-by: Pratik Karki <predatoramigo@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>