lint: Set remote_url variable in tools/lint-commits.

refer to:
zulip/zulip#17506

Author: LoopThrough-i-j

tools/lint-commits

@@ -9,11 +9,9 @@
 
 # The rules can be found in /.gitlint
 
-if [[ "
-$(git remote -v)
-" =~ '
-'([^[:space:]]*)[[:space:]]*(https://github\.com/|ssh://git@github\.com/|git@github\.com:)zulip/python-zulip-api(\.git|/)?\ \(fetch\)'
-' ]]; then
+remote_url='([^[:space:]]*)[[:space:]]*(https://github\.com/|ssh://git@github\.com/|git@github\.com:)zulip/python-zulip-api(\.git|/)?\ \(fetch\)'
+
+if [[ "$(git remote -v)" =~ $remote_url ]]; then
     range="${BASH_REMATCH[1]}/master..HEAD"
 else
     range="upstream/master..HEAD"