Report an error if the git ls-tree command fails in the content_aware_hash script (flutter#185170)

Without this, the script will output an incorrect hash and return a
successful exit code if "git ls-tree" fails.

See flutter#184376

Author: jason-simmons

bin/internal/content_aware_hash.sh

@@ -71,4 +71,8 @@ GIT_OPTS=()
 if [[ "$(git --version)" == *"Apple Git"* ]]; then
   GIT_OPTS=(-c core.multiPackIndex=false)
 fi
-git "${GIT_OPTS[@]}" -C "$FLUTTER_ROOT" ls-tree "$BASEREF" -- "${TRACKEDFILES[@]}" | git hash-object --stdin
+if ! HASH=$(set -o pipefail; git "${GIT_OPTS[@]}" -C "$FLUTTER_ROOT" ls-tree "$BASEREF" -- "${TRACKEDFILES[@]}" | git hash-object --stdin); then
+  >&2 echo "${0}: git error when generating Flutter content-aware hash"
+  exit 1
+fi
+echo "$HASH"