rebase -i: respect core.commentchar

Commit eff80a9 (Allow custom "comment char") introduced a custom comment
character for commit messages but did not teach git-rebase--interactive
to use it.

Change git-rebase--interactive to read core.commentchar and use its
value when generating commit messages and for the command list.

Signed-off-by: John Keeping <john@keeping.me.uk>

---
Changes since v1:
- use '\' as the comment character in tests
- use "git stripspace --strip-comments" where appropriate
- use "git stripspace --comment-lines" where appropriate
- use printf instead of echo when the string contains $comment_char
- quote $comment_char in case label