help: respect aliases

help: respect aliases

If we have an alias "foo" defined, then the help text for
"foo" (via "git help foo" or "git foo --help") now shows the
definition of the alias.

Before showing an alias definition, we make sure that there
is no git command which would override the alias (so that
even though you may have a "log" alias, even though it will
not work, we don't want to it supersede "git help log").

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>