perf/run: read GIT_PERF_REPO_NAME from perf.repoName

The GIT_PERF_REPO_NAME env variable is used in
the `aggregate.perl` script to set the 'environment'
field in the JSON Codespeed output.

Let's make it easy to set this variable by setting it
in a config file.

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

Author: chriscool

t/perf/run

@@ -137,6 +137,9 @@ run_subsection () {
 	get_var_from_env_or_config "GIT_PERF_MAKE_COMMAND" "perf" "makeCommand"
 	get_var_from_env_or_config "GIT_PERF_MAKE_OPTS" "perf" "makeOpts"
 
+	get_var_from_env_or_config "GIT_PERF_REPO_NAME" "perf" "repoName"
+	export GIT_PERF_REPO_NAME
+
 	GIT_PERF_AGGREGATING_LATER=t
 	export GIT_PERF_AGGREGATING_LATER