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.

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