builtin rebase: support --continue

builtin rebase: support --continue

This commit adds the option `--continue` which is used to resume
rebase after merge conflicts. The code tries to stay as close to
the equivalent shell scripts found in `git-legacy-rebase.sh` as
possible.

When continuing a rebase, the state variables are read from state_dir.
Some of the state variables are not actually stored there, such as
`upstream`. The shell script version simply does not set them, but for
consistency, we unset them in the builtin version.

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