doctool: Sort input filenames when generating indexes
doctool: Sort input filenames when generating indexes

Order of inputs may not be pre-sorted if the shell does not
sort arguments before passing them to commands. As shells are
not required to sort arguments, make sure they are sorted in
doctool itself.

dgray16/libreplan,dgray16/libreplan,LibrePlan/libreplan,PaulLuchyn/libreplan,skylow95/libreplan,poum/libreplan,poum/libreplan,PaulLuchyn/libreplan,Marine-22/libre,skylow95/libreplan,skylow95/libreplan,poum/libreplan,PaulLuchyn/libreplan,dgray16/libreplan,skylow95/libreplan,LibrePlan/libreplan,Marine-22/libre,LibrePlan/libreplan,LibrePlan/libreplan,LibrePlan/libreplan,LibrePlan/libreplan,Marine-22/libre,dgray16/libreplan,LibrePlan/libreplan,PaulLuchyn/libreplan,poum/libreplan,PaulLuchyn/libreplan,Marine-22/libre,PaulLuchyn/libreplan,skylow95/libreplan,skylow95/libreplan,poum/libreplan,dgray16/libreplan,Marine-22/libre,poum/libreplan,dgray16/libreplan,Marine-22/libre,PaulLuchyn/libreplan,dgray16/libreplan