completion: improve handling quoted paths in 'git ls-files's output

completion: improve handling quoted paths in 'git ls-files's output

If any pathname contains backslash, double quote, tab, newline, or any
control characters, 'git ls-files' and 'git diff-index' will enclose
that pathname in double quotes and escape those special characters
using C-style one-character escape sequences or \nnn octal values.
This prevents those files from being listed during git-aware path
completion, because due to the quoting they will never match the
current word to be completed.

Extend __git_index_files()'s 'awk' script to remove all that quoting
and escaping from unique path components, so even paths containing
(almost all) such special characters can be completed.

Paths containing newline characters are still an issue, though.  We
use newlines as separator character when filling the COMPREPLY array,
so a path with one or more newline will end up split to two or more
elements in COMPREPLY, basically breaking completion.  There is
nothing we can do about it without a significant performance hit, so
let's just ignore such paths for now.  As far as paths with newlines
are concerned, this isn't any different from the previous behavior,
because those paths were always omitted, though in the past they were
omitted because due to the quoting they didn't match the current word
to be completed.  Anyway, Bash's own filename completion (Meta-/) can
complete even those paths, if need be.

Note:

  - We don't dequote path components right away as they are coming in,
    because then we would have to dequote each directory name
    repeatedly, as many times as it appears in the input, i.e. as many
    times as the number of listed paths it contains.  Instead, we
    dequote them at the end, as we print unique path components.

  - Even when a directory name itself does not contain any special
    characters, it will still be quoted if any of its trailing path
    components do.  If a directory contains paths both with and
    without special characters, then the name of that directory will
    appear both quoted and unquoted in the output of 'git ls-files'
    and 'git diff-index'.  Consequently, we will add such a directory
    name to the deduplicating associative array twice: once quoted and
    once unquoted.

    This means that we have to be careful after dequoting a directory
    name, and only print it if we haven't seen the same directory name
    unquoted.

  - It would be wonderful if we could just pass '-z' to those git
    commands to output \0-separated unquoted paths, and use \0 as
    record separator in the 'awk' script processing their output...
    this patch would be so much simpler, almost trivial even.
    Unfortunately, however, POSIX and most 'awk' implementations don't
    support \0 as record separator (GNU awk does support it).

  - This patch makes the earlier change to list paths with
    'core.quotePath=false' basically redundant, because this could
    decode any \nnn-escaped non-ASCII character just fine, as well.
    However, I suspect that 'git ls-files' can deal with those
    non-ASCII characters faster than this updated 'awk' script; just
    in case someone is burdened with tons of pathnames containing
    non-ASCII characters.

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