If no SVN revision is found, return -1 instead of 0

git-svn-id: https://geany.svn.sourceforge.net/svnroot/geany/trunk@5982 ea778897-0a13-0410-b9d1-a72fbfd435f5

wscript

@@ -652,7 +652,7 @@ def _get_svn_rev(conf):
             pass
         except (IndexError, ValueError):
             Logs.pprint('RED', 'Unparseable revision number')
-    return 0
+    return -1
 
 
 def _load_intltool_if_available(conf):