Add get_var_from_env_or_config() to easily set variables
from a config file if they are defined there and not already set.
This can also set them to a default value if one is provided.
As an example, use this function to set GIT_PERF_REPEAT_COUNT
from the perf.repeatCount config option or from the default
value.
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>