Introduce a more subtle concept of validity when it comes to revision

indentifiers in IDF - the SCM function isValidRevision has been replaced
by a validateRevision() method which returns one of three states,
valid, invalid or ambiguous.
The source view can then act accordingly and display disambiguate view
for the latter, so the user can select for which revision he actually
wants to execute the requested action. Also, invalid revisions now lead
to another separate view, telling the user that it is invalid / does
not exist and pointing him optionally to the help page where he can read
further how to access his repository to push the first changes into.
(partially resolves issue 525)

src/IDF/templates/idf/source/invalid_revision.html

@@ -0,0 +1,17 @@
+{extends "idf/source/base.html"}
+{block docclass}yui-t2{assign $inError=true}{/block}
+{block body}
+
+<p>{blocktrans}The revision <b>{$commit}</b> is not valid or does not exist
+in this repository.{/blocktrans}</p>
+
+{if $isOwner or $isMember}
+{aurl 'url', 'IDF_Views_Source::help', array($project.shortname)}
+<p>{blocktrans}If this is a new repository, the reason for this error
+could be that you have not committed and / or pushed any change so far.
+In this case please take a look at the <a href="{$url}">Help</a> page
+how to access your repository.{/blocktrans}</p>
+{/if}
+
+{/block}
+