perf/run: add get_var_from_env_or_config()

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>

t/perf/perf-lib.sh

@@ -59,9 +59,6 @@ perf_results_dir=$TEST_OUTPUT_DIRECTORY/test-results
 mkdir -p "$perf_results_dir"
 rm -f "$perf_results_dir"/$(basename "$0" .sh).subtests
 
-if test -z "$GIT_PERF_REPEAT_COUNT"; then
-	GIT_PERF_REPEAT_COUNT=3
-fi
 die_if_build_dir_not_repo () {
 	if ! ( cd "$TEST_DIRECTORY/.." &&
 		    git rev-parse --build-dir >/dev/null 2>&1 ); then