help / completion: make "git help" do the hard work

The "help" builtin has been able to emit configuration variables since
e17ca926371 (completion: drop the hard coded list of config vars,
2018-05-26), but it hasn't produced exactly the format the completion
script wanted. Let's do that.

We got partway there in 2675ea1cc0f (completion: use 'sort -u' to
deduplicate config variable names, 2019-08-13) and
d9438873c4d (completion: deduplicate configuration sections,
2019-08-13), but after both we still needed some sorting,
de-duplicating and awk post-processing of the list.

We can instead simply do the relevant parsing ourselves (we were doing
most of it already), and call string_list_remove_duplicates() after
already sorting the list, so the caller doesn't need to invoke "sort
-u". The "--config-for-completion" output is the same as before after
being passed through "sort -u".

Then add a new "--config-sections-for-completion" option. Under that
output we'll emit config sections like "alias" (instead of "alias." in
the --config-for-completion output).

We need to be careful to leave the "--config-for-completion" option
compatible with users git, but are still running a shell with an older
git-completion.bash. If we e.g. changed the option name they'd see
messages about git-completion.bash being unable to find the
"--config-for-completion" option.

Such backwards compatibility isn't something we should bend over
backwards for, it's only helping users who:

 * Upgrade git
 * Are in an old shell
 * The git-completion.bash in that shell hasn't cached the old
   "--config-for-completion" output already.

But since it's easy in this case to retain compatibility, let's do it,
the older versions of git-completion.bash won't care that the input
they get doesn't change after a "sort -u".

While we're at it let's make "--config-for-completion" die if there's
anything left over in "argc", and do the same in the new
"--config-sections-for-completion" option.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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