[documentation]

Attribute   Description                          Required
file        The path to take the dirname of.     Yes
property    The name of the property to set.     Yes

The path must end with a file/directory name. These do not always exist.
Moreover, people wonder when they see 
&lt;<Dirname property="build.dir" file="./build/bin"&gt; 
and discover that the value of build.dir is just ./build instead of ./build/bin.

I propose the following enhancement:
If the specified file is a path that ends in a _directory_, the whole path will
be used as property's value.

However, this breaks compatibility. To avoid this it is possible to do either of
the following:
1) to add a boolean attribute "useWholePath",
2) to add an attribute "dir" that would complement "file" attribute.

Sorry for a long description and thank you in advance
    Kirill