head: reset stale partial line state on reused buffers

[can1357]

uutils head -n -N on non-seekable input can leak one trailing line because TakeAllLinesBuffer.partial_line is reused across fills and can stay true after a later terminated chunk. GNU recomputes tail state from current bytes each read; this fix resets the flag in fill_buffer and adds a deterministic regression covering the non-seekable code path.

Reproduction Steps

t=$(mktemp -u)
mkfifo "$t"
{ exec 3>"$t"; printf 'A' >&3; sleep 0.05; printf '\nB\nC' >&3; sleep 0.05; printf 'D\n' >&3; exec 3>&-; } &
head -n -1 <"$t" | od -An -tx1 -v
wait
rm "$t"
# Expected (GNU): 41 0a 42 0a
# Actual (uutils): 41 0a 42 0a 43 44 0a

Impact

Pipelines that rely on head -n -N for tail elision on non-seekable streams can silently emit data that should be suppressed. This is a correctness issue.