Makefile: dedup list of files obtained from ls-files

Since 33533975 ("Makefile: ask "ls-files" to list source files if
available", 2011-10-18), we optionally asked "ls-files" to list the
source files that ought to exist, as a faster approximation for
"find" on working tree files.

This works reasonably well, except that it ends up listing the same
path multiple times if the index is unmerged.  Because the original
use of this construct was to name files to run etags over, and the
etags command happily takes the same filename multiple times without
causing any harm, there was no problem (other than perhaps spending
slightly more cycles, but who cares how fast the TAGS file gets
updated).

We however recently added a similar call to "ls-files" to list *.h
files, instead of using "find", in 92b88eba ("Makefile: use `git
ls-files` to list header files, if possible", 2019-03-04).  In this
new use of "ls-files", the resulting list $(LIB_H) is used for,
among other things, generating the header files to run hdr-check
target, and the duplicate unfortunately becomes a true problem.  It
causes $(MAKE) to notice that there are multiple %.hco targets and
complain.

Let the resulting list consumed by $(sort), which deduplicates,
to fix this.

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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