dim: Replace git commit --amend from dim_apply with dimrc option

Introduce DIM_POST_APPLY_ACTION to dimrc that allows the user to specify
a command to be run after a patch is applied. Use eval so enviroment
variables can be overriden with the option. For example:

DIM_POST_APPLY_ACTION="EDITOR=\"gedit -w\" git commit --amend"

v2: Initialize variable with default value.
    Fix dimrc.sample to match default value.

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>