phab-refresh: extract the comment in a variable

This make the script more readable and it is about to become more complex.

Differential Revision: https://phab.mercurial-scm.org/D10463

contrib/heptapod-ci.yml

@@ -51,8 +51,10 b' rust-cargo-test-py3:'
 
 phabricator-refresh:
     stage: phabricator
+    variables:
+      DEFAULT_COMMENT: ":white_check_mark: refresh by Heptapod after a successful CI run (:octopus: :green_heart:)"
     script:
-      - "./contrib/phab-refresh-stack.sh --comment \":white_check_mark: refresh by Heptapod after a successful CI run (:octopus: :green_heart:)\""
+      - "./contrib/phab-refresh-stack.sh --comment \"$DEFAULT_COMMENT\""
 
 test-py2:
     <<: *runtests