pre-commit.entrypoint.sh: readd unset option

While it is true that there are no variables that can be unset
@delliot, who created it, it prefers to have it turned on:
#5540 (comment)

Signed-off-by: Filip Kobierski <fkobi@fsfe.org>

pre-commit.entrypoint.sh

@@ -1,5 +1,5 @@
 #!/bin/sh
-set -e
+set -eu
 
 # Get the changed files
 target_branch="${1:-master}"