maint: try to avoid unwanted chatter in ChangeLog

* bootstrap.conf: Do not alter ChangeLog if it already exists.

bootstrap.conf

@@ -68,7 +68,7 @@ fi
 test -d m4 || mkdir m4
 test -d $source_base || mkdir $source_base
 
-cat > ChangeLog <<EOT
+test -f ChangeLog || cat > ChangeLog <<EOT
 This file is a placeholder. It will be replaced with the actual ChangeLog
 by make dist.  Run make ChangeLog if you wish to create it earlier.
 EOT