git-completion.bash: separate some commands onto their own line

git-completion.bash: separate some commands onto their own line

In e94fb44042 (git-completion.bash: pass $__git_subcommand_idx from
__git_main(), 2021-03-24), a line was introduced which contained
multiple statements. This is difficult to read so break it into multiple
lines.

While we're at it, follow this convention for the rest of the
__git_main() and break up lines that contain multiple statements.

Signed-off-by: Denton Liu <redacted>
Signed-off-by: Junio C Hamano <redacted>