Use $(...) notation instead of legacy backticked `...` in shell script

Reference:
- https://github.com/koalaman/shellcheck/wiki/SC2006

lib/support/init.d/gitlab

@@ -67,7 +67,7 @@ gitaly_log="$app_root/log/gitaly.log"
 test -f /etc/default/gitlab && . /etc/default/gitlab
 
 # Switch to the app_user if it is not they who are running the script.
-if [ `whoami` != "$app_user" ]; then
+if [ $(whoami) != "$app_user" ]; then
   eval su - "$app_user" -c $(echo \")$shell_path -l -c \'$0 "$@"\'$(echo \"); exit;
 fi
 

scripts/lint-conflicts.sh

 #!/bin/sh
 
-output=`git grep -En '^<<<<<<< '`
+output=$(git grep -En '^<<<<<<< ')
 echo $output
 test -z "$output"