git-rebase-hash: new face

Use the same value as for `magit-log-sha1', i.e. some reddish color.
git-rebase-description-face: inherit from default

Set the default value to nil, i.e. inherit from `default'.  Previously
this did inherit from `font-lock-comment-face', which isn't so good
because it is for the description of commits that are *not* commented
out, *and* is other text in the buffer which are comments and also use
the respective face.
git-rebase-{description,killed-action}: drop -face suffix

Face symbols shouldn't end with `-face', Font-Lock faces are an
exception not the rule.  Define aliases.
magit-rebase-description: move definition
git-rebase-mode.el: new subsections

Split the "Options" section into subsections "Variables" and "Faces".
git-rebase-mode.el: improve some doc-strings
git-commit-mode.el: improve some doc-strings
git-rebase-mode-map: explicitly inherit from special-mode-map

I find it confusing when `define-derived-mode' does that behind my back.