Fix error in dirname computation in TreeView

In TreeView:on_mouse_pressed() we need to find the directory a
relative filename belongs to from its absolute filename.
The code was using string.find to locate the relative filename
within the absolute path but in some very specific cases we can
find a pattern which is not the right-most one leading to a
wrong directory name.

Fix the error by adding a loop to make sure we find the right-most
match. The standard Lua library has not a string.rfind to make a
reverse search.

Close #275

data/plugins/treeview.lua

@@ -222,7 +222,14 @@ function TreeView:on_mouse_pressed(button, x, y, clicks)
     else
       if core.project_files_limit and not hovered_item.expanded then
         local filename, abs_filename = hovered_item.filename, hovered_item.abs_filename
-        local index = string.find(abs_filename, filename, 1, true)
+        local index = 0
+        -- The loop below is used to find the first match starting from the end
+        -- in case there are multiple matches.
+        while index and index + #filename < #abs_filename do
+          index = string.find(abs_filename, filename, index + 1, true)
+        end
+        -- we assume here index is not nil because the abs_filename must contain the
+        -- relative filename
         local dirname = string.sub(abs_filename, 1, index - 2)
         core.scan_project_folder(dirname, filename)
         self:invalidate_cache(dirname)