Fix case-insensitive completion of buffer names

Fix case-insensitive completion of buffer names

* test/src/minibuf-tests.el (test-try-completion-ignore-case):
New test, suggested by Stefan Monnier <monnier@iro.umontreal.ca>.

* src/minibuf.c (Ftry_completion): Don't treat strings that
are identical but for the case as if they were identical for
the purposes of not counting the same string twice.  This
fixes case-insensitive completion when all the candidates are
identical but for the letter-case.  (Bug#11339)