Fixed a bug that prevented finding nested subfiles

[zalgo3]

When we use the package \subfiles and if we load nested files with relative paths, e.g., if we have more \subfiles in a file read by \subfile, which is in some different directory, latexdiff cannot deal with the inner \subfile.

This is due to the fact that when expanding the \subfile, the argument to flatten is $filename, i.e. the current path. https://github.com/ftilmann/latexdiff/blob/9d3a00aec26984018fdca1086ddff97af9678541/latexdiff#L1795 By changing $filename to $fullfile, i.e., the path of the file to read, the program can find the file even when there is nesting.

[ftilmann]

Thanks for fixing finding this issue and fixing it.