help.c: refactor drop_prefix() to use a "switch" statement"

Refactor the drop_prefix() function in in help.c to make it easier to
strip prefixes from categories that aren't "CAT_guide". There are no
functional changes here, by doing this we make a subsequent functional
change's diff smaller.

As before we first try to strip "git-" unconditionally, if that works
we'll return the stripped string. Then we'll strip "git" if the
command is in "CAT_guide".

This means that we'd in principle strip "git-foo" down to "foo" if
it's in CAT_guide. That doesn't make much sense, and we don't have
such an entry in command-list.txt, but let's preserve that behavior
for now.

While we're at it remove a stray newline that had been added after the
"return name;" statement.

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