Documentation: No argument of ALLOC_GROW should have side-effects

Documentation: No argument of ALLOC_GROW should have side-effects

The explanatory comment before the definition of ALLOC_GROW carefully
lists arguments that will be used more than once and thus cannot have
side-effects; a lazy reader might conclude that the arguments not
listed are used only once and side effects safe.

Correct it to list all three arguments, avoiding this confusion.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>