teach more commands to respect git-rebase-auto-advance, default to t
remove git-rebase-define-action

I don't see the point in a macro whose only purpose
is to cut the line count by half-a-dozen lines.

Also add basic doc-strings to the affected commands.
git-rebase-hash: use same color as for magit-hash

Because `magit' depends on `git-rebase-mode', not the
other way around, we cannot inherit from `magit-hash'.
git-rebase-killed-action: set value for all classes
git-rebase-mode-menu: cosmetics
git-rebase-undo: improve doc-string
git-rebase-move-line-down: remove pointless comment

Hello Captain Obvious.
git-rebase-backward-line: cosmetics
magit-rebase-exec: change semantics

When there is a command on the current line then always edit it.
Even when it is commented out, like before that is uncommented
but the prompting also happens.

A prefix argument now forces prompting for a new command, instead
of editing the command on the current line.  So the behavior of
the prefix argument is more or less reverses.

This also allows the removal of the utility functions
`git-rebase-looking-at-exec' and `git-rebase-looking-at-killed-exec'.
refactor rebase commands, slightly changing their behavior

Refactor the commands `git-rebase-move-line-up',
`git-rebase-move-line-down', `git-rebase-kill-line',
`git-rebase-rebase-exec', `git-rebase-show-commit' and as well as the
utility function `git-rebase-edit-line', which is used by many of the
other commands.

The behavior of these commands changes slightly, in that they no longer
try to avoid moving point to the beginning of the line.  Previously
some of them did, while others did not.  Now they consistently do what
I prefer (move to bol) and which is also easier to implement.

Thanks to that we can now remove the utility functions
`git-rebase-looking-at-action' and
`git-rebase-looking-at-action-or-exec', and replace the constants
`git-rebase-action-line-re', `git-rebase-exec-line-re' and
`git-rebase-dead-line-re' with just `git-rebase-line'.  Also define
some regexps directly in `git-rebase-mode-font-lock-keywords'.
rename git-rebase-edit-line to git-rebase-set-action
Merge branch 'n/cleanup-rebase' into next