completion: repair config completion for Zsh

completion: repair config completion for Zsh

Commit 1e0ee4087e (completion: add and use
__git_compute_first_level_config_vars_for_section, 2024-02-10) uses an
indirect variable syntax that is only valid for Bash, but the Zsh
completion code relies on the Bash completion code to function. Zsh
supports a different indirect variable expansion using ${(P)var}, but in
`emulate ksh` mode does not support Bash's ${!var}.

This manifests as completing strange config options like
"__git_first_level_config_vars_for_section_remote" as a choice for the
command line

    git config set remote.

Using Zsh's C-x ? _complete_debug widget with the cursor at the end of
that command line captures a trace, in which we see (some details
elided):

    +__git_complete_config_variable_name:7> __git_compute_first_level_config_vars_for_section remote
     +__git_compute_first_level_config_vars_for_section:7> local section=remote
     +__git_compute_first_level_config_vars_for_section:7> __git_compute_config_vars
      +__git_compute_config_vars:7> test -n $'add.ignoreErrors\nadvice.addEmbeddedRepo\nadvice.addEmptyPathspec\nadvice.addIgnoredFile[…]'
     +__git_compute_first_level_config_vars_for_section:7> local this_section=__git_first_level_config_vars_for_section_remote
     +__git_compute_first_level_config_vars_for_section:7> test -n __git_first_level_config_vars_for_section_remote
    +__git_complete_config_variable_name:7> local this_section=__git_first_level_config_vars_for_section_remote
    +__git_complete_config_variable_name:7> __gitcomp_nl_append __git_first_level_config_vars_for_section_remote remote. '' ' '
     +__gitcomp_nl_append:7> __gitcomp_nl __git_first_level_config_vars_for_section_remote remote. '' ' '
      +__gitcomp_nl:7> emulate -L zsh
      +__gitcomp_nl:7> compset -P '*[=:]'
      +__gitcomp_nl:7> compadd -Q -S ' ' -p remote. -- __git_first_level_config_vars_for_section_remote

We perform the test for __git_compute_config_vars correctly, but the
${!this_section} references are not expanded as expected.

Instead, portably expand indirect references through the new
__git_indirect. Contrary to some versions you might find online [1],
this version avoids echo non-portabilities [2] [3] and correctly quotes
the indirect expansion after eval (so that the result is not split or
globbed before being handed to printf).

[1]: https://unix.stackexchange.com/a/41409/301073
[2]: https://askubuntu.com/questions/715765/mysterious-behavior-of-echo-command#comment1056038_715769
[3]: https://mywiki.wooledge.org/CatEchoLs

The following demo program demonstrates how this works:

    b=1
    indirect() {
      eval printf '%s' "\"\$$1\""
    }
    f() {
      # Comment this out to see that it works for globals, too. Or, use
      # a value with spaces like '2 3 4' to see how it handles those.
      local b=2
      local a=b
      test -n "$(indirect $a)" && echo nice
    }
    f

When placed in a file "demo", then both
    bash -x demo
and
    zsh -xc 'emulate ksh -c ". ./demo"' |& tail
provide traces showing that "$(indirect $a)" produces 2 (or 1, with the
global, or "2 3 4" as a single string, etc.).

Signed-off-by: D. Ben Knoble <ben.knoble+github@gmail.com>
Acked-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>