reset: reorder wildcard pathspec conditions

reset: reorder wildcard pathspec conditions

Rearrange conditions in method determining whether index expansion is
necessary when a pathspec is specified for `git reset`, placing less
expensive condition first. Additionally, add details & examples to related
code comments to help with readability.

Helped-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Victoria Dye <vdye@github.com>
Reviewed-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>