perf/run: add '--config' option to the 'run' script

It is error prone and tiring to use many long environment
variables to give parameters to the 'run' script.

Let's make it easy to store some parameters in a config
file and to pass them to the run script.

The GIT_PERF_CONFIG_FILE variable will be set to the
argument of the '--config' option. This variable is not
used yet. It will be used in a following commit.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/perf/run

@@ -2,9 +2,14 @@
 
 case "$1" in
 	--help)
-		echo "usage: $0 [other_git_tree...] [--] [test_scripts]"
+		echo "usage: $0 [--config file] [other_git_tree...] [--] [test_scripts]"
 		exit 0
 		;;
+	--config)
+		shift
+		GIT_PERF_CONFIG_FILE=$(cd "$(dirname "$1")"; pwd)/$(basename "$1")
+		export GIT_PERF_CONFIG_FILE
+		shift ;;
 esac
 
 die () {