pre-commit: only run uncrustify if changed files are detected

This adds the "no-run-if-empty" flag to the xargs invocation.  Otherwise,
if git diff names no files, the command is run once with no agument
specified which leads to running on a default list of files that appears
to include ignored files and files in submodules.

I am uneasy about how this works (it means that `pre-commit run --all`
doesn't actually check all files) but that's a separate issue.

Author: jepler

.pre-commit-config.yaml

@@ -21,6 +21,6 @@ repos:
         language: system
     -   id: formatting
         name: Formatting
-        entry: sh -c "git diff --staged --name-only | xargs python3 tools/codeformat.py"
+        entry: sh -c "git diff --staged --name-only | xargs -r python3 tools/codeformat.py"
         types_or: [c, python]
         language: system