completion tests: re-source git-completion.bash in a subshell

completion tests: re-source git-completion.bash in a subshell

Change tests of git-completion.bash that re-source it to do so inside
a subshell. Re-sourcing it will clobber variables it sets, and in the
case of the "GIT_COMPLETION_SHOW_ALL=1" test added in
ca2d62b7879 (parse-options: don't complete option aliases by default,
2021-07-16) change the behavior of the completion persistently.

Aside from the addition of "(" and ")" on new lines this is an
indentation-only change, only the "(" and ")" lines are changed under
"git diff -w".

So let's change that test, and for good measure do the same for the
three tests that precede it, which were added in
8b0eaa41f23 (completion: clear cached --options when sourcing the
completion script, 2018-03-22). The may not be wrong, but doing this
establishes a more reliable pattern for future tests, which might use
these as a template to copy.

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