Adds a script to delete old tags (#7158)

scripts/git/prune_old_tags.sh

diff --git a/scripts/git/prune_old_tags.sh b/scripts/git/prune_old_tags.sh
new file mode 100755
index 0000000..627005a
--- /dev/null
+++ b/scripts/git/prune_old_tags.sh
@@ -0,0 +1,82 @@
+#!/bin/bash
+
+# Copyright 2021 The IREE Authors
+#
+# Licensed under the Apache License v2.0 with LLVM Exceptions.
+# See https://llvm.org/LICENSE.txt for license information.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+# Deletes all but the input number of snapshot tags and corresponding releases,
+# starting with the oldest tags.
+#
+# Delete all but the 100 newest tags from the repository at the UPSTREAM_REMOTE
+# environment variable:
+#   ./prune_old_tags.sh 100
+#
+# List the tags that would be deleted when not performing a dry run:
+#   ./prune_old_tags.sh 100 DRY_RUN
+
+set -euo pipefail
+
+KEEP_TAGS_COUNT="$1"
+DRY_RUN="${2:-}"
+UPSTREAM_REMOTE="${UPSTREAM_REMOTE:-upstream}"
+
+DRY_RUN="${DRY_RUN//[-_]/}"
+DRY_RUN="${DRY_RUN^^}"
+
+if [[ -n "${DRY_RUN}" ]] && [[ "${DRY_RUN}" != "DRYRUN" ]]; then
+  echo "Unexpected argument '${2}'. Should be DRYRUN (case and punctuation insensitive) if present"
+  exit 1
+fi
+
+if [[ -z "${KEEP_TAGS_COUNT}" ]]; then
+  echo "Must specify the number of tags to keep"
+  exit 1
+fi
+
+if [[ "${KEEP_TAGS_COUNT}" -le 0 ]]; then
+  echo "Cannot delete all remaining tags"
+  exit 1
+fi
+
+git fetch --all --tags
+TAGS=($(git for-each-ref --sort=creatordate --format '%(refname:short)' refs/tags))
+
+# Filter out tags that were not created by snapshot releases.
+for TAG_INDEX in "${!TAGS[@]}"; do
+  if [[ ! ${TAGS[TAG_INDEX]} =~ ^snapshot-* ]]; then
+    unset 'TAGS[TAG_INDEX]'
+  fi
+done
+
+TOTAL_TAGS_COUNT=${#TAGS[@]}
+if [[ "${KEEP_TAGS_COUNT}" -ge "${TOTAL_TAGS_COUNT}" ]]; then
+  echo "Only ${TOTAL_TAGS_COUNT} tags exist, so nothing to delete"
+  exit 1
+fi
+
+DELETE_TAGS_COUNT=$((TOTAL_TAGS_COUNT-KEEP_TAGS_COUNT))
+DELETE_TAGS=${TAGS[@]::$DELETE_TAGS_COUNT}
+
+echo "${TOTAL_TAGS_COUNT} snapshot tags available"
+echo "${DELETE_TAGS_COUNT} tags will be deleted"
+
+if [[ "${DRY_RUN}" == "DRYRUN" ]]; then
+  echo "Dry run mode, these tags would be deleted:"
+  for TAG in ${DELETE_TAGS[@]}; do
+    echo "${TAG}"
+  done
+  exit 0
+fi
+
+read -p "Continue? (y/n) " -n 1 -r
+echo
+if [[ ! "${REPLY}" =~ ^[Yy]$ ]]
+then
+  echo "Exiting"
+  exit 0
+fi
+
+git tag -d ${DELETE_TAGS[@]}
+git push "${UPSTREAM_REMOTE?}" -d ${DELETE_TAGS[@]}