parse-options: don't complete option aliases by default

parse-options: don't complete option aliases by default

Since 'OPT_ALIAS' was created in 5c387428f1 (parse-options: don't emit
"ambiguous option" for aliases, 2019-04-29), 'git clone
--git-completion-helper', which is used by the Bash completion script to
list options accepted by clone (via '__gitcomp_builtin'), lists both
'--recurse-submodules' and its alias '--recursive', which was not the
case before since '--recursive' had the PARSE_OPT_HIDDEN flag set, and
options with this flag are skipped by 'parse-options.c::show_gitcomp',
which implements 'git <cmd> --git-completion-helper'.

This means that typing 'git clone --recurs<TAB>' will yield both
'--recurse-submodules' and '--recursive', which is not ideal since both
do the same thing, and so the completion should directly complete the
canonical option.

At the point where 'show_gitcomp' is called in 'parse_options_step',
'preprocess_options' was already called in 'parse_options', so any
aliases are now copies of the original options with a modified help text
indicating they are aliases.

Helpfully, since 64cc539fd2 (parse-options: don't leak alias help
messages, 2021-03-21) these copies have the PARSE_OPT_FROM_ALIAS flag
set, so check that flag early in 'show_gitcomp' and do not print them,
unless the user explicitely requested that *all* completion be shown (by
setting 'GIT_COMPLETION_SHOW_ALL'). After all, if we want to encourage
the use of '--recurse-submodules' over '--recursive', we'd better just
suggest the former.

The only other options alias is 'log' and friends' '--mailmap', which is
an alias for '--use-mailmap', but the Bash completion helpers for these
commands do not use '__gitcomp_builtin', and thus are unnaffected by
this change.

Test the new behaviour in t9902-completion.sh. As a side effect, this
also tests the correct behaviour of GIT_COMPLETION_SHOW_ALL, which was
not tested before. Note that since '__gitcomp_builtin' caches the
options it shows, we need to re-source the completion script to clear
that cache for the second test.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>