builtin rebase: support --onto

builtin rebase: support --onto

The `--onto` option is important, as it allows to rebase a range of
commits onto a different base commit (which gave the command its odd
name: "rebase").

This commit introduces options parsing so that different options can
be added in future commits.

Note: As this commit introduces to the parse_options() call (which
"eats" argv[0]), the argc is now expected to be lower by one after this
patch, compared to before this patch: argv[0] no longer refers to the
command name, but to the first (non-option) command-line parameter.

Signed-off-by: Pratik Karki <predatoramigo@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>