…h not be non-empty.

This commit was originally introduced as part of #41420 and then
reverted with the rest of that PR. However there was no reason to revert
his particular commit.

Excludes and includes are implemented by passing the parameters to the
respective flags of `filterdiff`. Those were passed unescaped until now.
Since those flags expect patterns (similar to shell globs), something
like `/some/path/*` might be used to exclude or include all files in
some path. Without escaping the shell would expand the `*`, leading to
unexpected behaviour.

Since this is probably never the desired case and has led to actual
issues, see the comments at:

af1313e

This might also happen when pulling a patch from GitHub or a similar web
interface without explicitly selecting the "raw" format.

fetchpatch: quote excludes