Fix the inode count in pct-shrink-rootfs

GNU df refuses -i together with --output, so the script stopped right
after mounting the container ("options -i and --output are mutually
exclusive"). The iused field needs no -i: use df --output=iused.

The df stub in the tests accepted the combination and hid the bug; it now
rejects it the way GNU df does.

Generated with AI

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_pct_shrink.py

@@ -20,7 +20,9 @@ STUBS = {
     "ha-manager": 'echo "${STUB_HA_OUT:-}"',
     "pvesr": 'echo "${STUB_PVESR_OUT:-}"',
     "df": """echo "df $*" >> "$STUB_LOG"
-if [[ "$*" == *-i* ]]; then echo IUsed; echo "${STUB_IUSED:-1000}"
+# like GNU df: -i and --output cannot be combined
+if [[ " $* " == *" -i "* && "$*" == *--output* ]]; then echo "df: options -i and --output are mutually exclusive" >&2; exit 1; fi
+if [[ "$*" == *--output=iused* ]]; then echo IUsed; echo "${STUB_IUSED:-1000}"
 else echo Used; echo "${STUB_USED:-1073741824}"; fi""",
     "rsync": """echo "rsync $*" >> "$STUB_LOG"
 [[ -z ${STUB_RSYNC_FAIL:-} ]]""",