--8<----8<----8<----8<----8<-- begin -->8---->8---->8---->8---->8----
svn: warning: W200005: 'C:\blah\blah\blah\\trunk\ª.tex' is not under version control
svn: E200009: Could not cat all targets because some targets are not versioned
svn: E200009: Illegal target for the requested operation
--8<----8<----8<----8<----8<--  end  -->8---->8---->8---->8---->8----

looking more carefully at things, and having inspected the C code of
function call-process, I realized that the variable
coding-system-for-write has some importance, and tracing this I realized
that when the first svn command is called the variable
coding-system-for-write is nil, while when the second svn command is
called it is 'no-conversion, which makes it fail.