Prevent duplicating active_bugs.csv header (#474)

[agodio]

If a project has already been promoted to the main Defects4J database, if (-e "$CORE_DIR/Project/$PID.pm") { evaluates to true at initialize-project-and-collect-issues.pl:180, active_bugs.csv ends up with a duplicated header: one from the just mined bugs ($COMMIT_DB_FILE) and the other from the complement computed by grep -vFf $COMMIT_DB_FILE.orig $COMMIT_DB_FILE, since $COMMIT_DB_FILE.orig lacks such header.