t9902-completion: exercise __git_complete_index_file() directly

t9902-completion: exercise __git_complete_index_file() directly

The tests added in 2f271cd9cf (t9902-completion: add tests
demonstrating issues with quoted pathnames, 2018-05-08) and in
2ab6eab4fe (completion: improve handling quoted paths in 'git
ls-files's output, 2018-03-28) have a few shortcomings:

  - All these tests use the 'test_completion' helper function, thus
    they are exercising the whole completion machinery, although they
    are only interested in how git-aware path completion, specifically
    the __git_complete_index_file() function deals with unusual
    characters in pathnames and on the command line.

  - These tests can't satisfactorily test the case of pathnames
    containing spaces, because 'test_completion' gets the words on the
    command line as a single argument and it uses space as word
    separator.

  - Some of the tests are protected by different FUNNYNAMES_* prereqs
    depending on whether they put backslashes and double quotes or
    separator characters (FS, GS, RS, US) in pathnames, although a
    filesystem not allowing one likely doesn't allow the others
    either.

  - One of the tests operates on paths containing '|' and '&'
    characters without being protected by a FUNNYNAMES prereq, but
    some filesystems (notably on Windows) don't allow these characters
    in pathnames, either.

Replace these tests with basically equivalent, more focused tests that
call __git_complete_index_file() directly.  Since this function only
looks at the current word to be completed, i.e. the $cur variable, we
can easily include pathnames containing spaces in the tests, so use
such pathnames instead of pathnames containing '|' and '&'.  Finally,
use only a single FUNNYNAMES prereq for all kinds of special
characters.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>