[WIP/RFC] Remove third argument of concat_fnames().

Running `$ ag concat_fnames` reveals that the third argument is `true`,
except for one instance where vim_strcat() can be used instead.
:+1:
If `@param` is left out, doxygen will output a warning to point out places where documentation is missing. If the comment is empty, documentation is still missing, but we don't get notified about it.