help: convert git_cmd to page in one place

help: convert git_cmd to page in one place

Depending on the chosen format of help pages, git-help uses function
show_man_page, show_info_page, or show_html_page.  The first thing all
three functions do is to convert given `git_cmd` to a `page` using
function cmd_to_page.

Move the common part of these three functions to function cmd_help to
avoid code duplication.

Signed-off-by: Andrei Rybak <redacted>
Reviewed-by: Felipe Contreras <redacted>
Acked-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Junio C Hamano <redacted>