web commit by http://jeremie.koenig.myopenid.com/: add a plugin page for "showdiff"

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+[[template id=plugin name=showdiff author="[[JeremieKoenig]]"]]
+[[tag type/useful]]
+
+This plugin, which can be downloaded
+[here](http://www.jk.fr.eu.org/ikiwiki/showdiff.pm),
+adds a "Show Diff" button to the page edition template.
+When clicked, a diff between the stored page and provided content
+is shown in the "Page Preview" area.
+
+It depends on
+[this patch](http://www.jk.fr.eu.org/ikiwiki/pluggable_editpage_buttons.diff),
+which does the following:
+
+  * add a title to the editpage form;
+  * pass a reference to the list of buttons to the formbuilder_setup
+    hooks, so we can add ours;
+  * relax asumption about the possible submit values (use "Save Page"
+    explicitly);
+  * de-hardcode the submit buttons from the editpage template
+    (was this intended to work around something?).
+
+## Problems
+
+No special handling is done of concurrent edits: changes introduced
+independently will show up in the requested diff, although they will
+be merged when the page is saved. I suspect even detecting this case
+would require changes in the RCS backends.