Fix COMMIT_ID in regression_test.sh (#9047)

Summary:
https://github.com/facebook/rocksdb/issues/8031 broke internal tests. This should fix but also preserve
the intended capability of getting git commit id when hg not used

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9047

Test Plan: already broken ¯\\_(ツ)_/¯

Reviewed By: zhichao-cao

Differential Revision: D31732198

Pulled By: pdillinger

fbshipit-source-id: 7dba8531ddca55a6de5e04978a1a1601aae4cee9

tools/regression_test.sh

@@ -158,7 +158,7 @@ function init_arguments {
 
   current_time=$(date +"%F-%H:%M:%S")
   RESULT_PATH=${RESULT_PATH:-"$1/results/$current_time"}
-  COMMIT_ID=`git log | head -n1 | cut -c 8-`
+  COMMIT_ID=`hg id -i 2>/dev/null || git rev-parse HEAD 2>/dev/null || echo 'unknown'`
   SUMMARY_FILE="$RESULT_PATH/SUMMARY.csv"
 
   DB_PATH=${3:-"$1/db"}