tools/gen-changelog.sh: Exclude "-preview" tags from generated log.

Signed-off-by: Damien George <damien@micropython.org>

tools/gen-changelog.sh

@@ -2,7 +2,7 @@
 
 echo "MicroPython change log"
 
-for t in $(git tag | grep -v v1.0-rc1 | sort -rV); do
+for t in $(git tag | grep -v -- '-rc1\|-preview' | sort -rV); do
     echo ''
     echo '========'
     echo ''