Goal: Web interface to allow reverting of changes.

Interface: 

At least at first, it will be exposed via the recentchanges
page, with revert icons next to each change. We may want a dynamic 
per-page interface that goes back more than 100 changes later.

Limiting assumptions:

* No support for resolving conflicts in reverts; such a revert would just
  fail and not happen.
* No support for reset-to-this-point; initially the interface would only
  revert a single commit, and if a bunch needed to go, the user would have
  to drive that one at a time.

Implementation plan: 

* `rcs_revert` hook that takes a revision to revert.
* CGI: `do=revert&rev=foo`
* recentchanges plugin adds above to recentchanges page
* prompt user to confirm (to avoid spiders doing reverts),
  check that user is allowed to make the change, commit reversion,
  and refresh site.

Peter Gammie has done an initial implementation of the above.
[[!template id=gitbranch branch=peteg/revert author="[[peteg]]"]]

>> It is on a separate branch now. --[[peteg]]

> Review: --[[Joey]] 
> 
> The revert commit will not currently say what web user did the revert.
> This could be fixed by doing a --no-commit revert first and then using
> rcs_commit_staged.
>> Fixed, I think. --[[peteg]]
>
> So I see one thing I completly forgot about is `check_canedit`. Avoiding users
> using reverting to make changes they would normally not be allowed to do is
> tricky. I guess that a easy first pass would be to only let admins do it.
> That would be enough to get the feature out there..
> 
> I'm thinking about having a `rcs_preprevert`. It would take a rev and look
> at what changes reverting it would entail, and return the same data
> structure that `rcs_recieve` does. This could be done by using `git revert
> --no-commit`, and then examining the changes, and then `git reset` to drop
> them.
>> We can use the existing `git_commit_info` with the patch ID - no need to touch the working directory. -- [[peteg]]
> 
> Then the code that is currently in IkiWiki/Receive.pm, that calls
> `check_canedit` and `check_canremove` to test the change, can be
> straightforwardly refactored out, and used for checking reverts too.
>> Wow, that was easy. :-) -- [[peteg]]
> 
> (The data from `rcs_preprevert` could also be used for a confirmation
> prompt -- it doesn't currently include enough info for diffs, but at
> least could have a list of changed files.)
>
> Note that it's possible for a git repo to have commits that modify wiki
> files in a subdir, and code files elsewhere. `rcs_preprevert` should
> detect changes outside the wiki dir, and fail, like `rcs_receive` does.
>> Taken care of by refactoring `rcs_receive` in `git.pm`
>> I've tested it lightly in my single-user setup. It's a little nasty that the `attachment` plugin
>> gets used to check whether attachments are allowed -- there really should be a hook for that.
>>
>> Please look it over and tell me what else needs fixing... -- [[peteg]]

>>> I have made my own revert branch and put a few fixes in there
>>> [[!template id=gitbranch branch=origin/revert author="[[joey]]"]]
>>> (and fixed all the indention..). Issues I noticed but have not gotten
>>> to: --[[Joey]] 
>>>> Please change the git pointer above, then. I will work on your branch. -- [[peteg]]

>>> * Would it be better for `rcs_revert` to not commit, and
>>>   `rcs_commit_staged` to then be used? This would work for git, but
>>>   maybe other RCSs would be problimatic. It would simplifiy the
>>>   interface and allow for future mulitple-revert interfaces.
>>> * I quite don't understand why one caller of `git_parse_changes`
>>>   needs it to chdir, and not the other one. It's running
>>>   in the same git repo either way, and git doesn't need
>>>   `git show` to run in a subdir at all..
>>>> I was aping (preserving) what was already there. I don't understand what you say about `git show` - it must run under $srcdir, surely? And empirically the CGI process wasn't in the right place. By all means simplify that. -- [[peteg]]

>>> * Probably needs to untaint the revs passed in.
>>> * Seems backwards for `rcs_preprevert` to import and
>>>   use `IkiWiki::Receive`.
>>>> Indeed. This is saying that the checking code in IkiWiki::Receive is in the wrong place. I think it would be better to set up some general hooks and shuffle it into a plugin, for then other plugins that maintain special files in the repo can have a say about validity. -- [[peteg]]