grep - -B/-C before-context count is rounded up to a power of two

Found by the differential fuzzer with args grep ["-e", "l", "-B", "3"].

-B N (and the before half of -C N) should print exactly N lines of context before each
match. uu_grep instead prints up to the next power of two: -B 3 prints 4 before-context
lines, -B 5/-B 6/-B 7 print 8. Values that are already powers of two (1, 2, 4, 8)
are correct, which is what makes the bug intermittent.

Reproduction

Input is ten numbered lines 01..10 followed by a matching line MM.

Rust (incorrect)

$ printf '01\n02\n03\n04\n05\n06\n07\n08\n09\n10\nMM\n' | ./target/release/grep -e MM -B 3
# Output:
# 07
# 08
# 09
# 10
# MM
# (four before-context lines instead of three)

GNU (correct)

$ printf '01\n02\n03\n04\n05\n06\n07\n08\n09\n10\nMM\n' | LC_ALL=C /usr/bin/grep -e MM -B 3
# Output:
# 08
# 09
# 10
# MM
# (three before-context lines)

Before-context line counts observed for -B 1..7 (match has 10 preceding lines):

The counts follow 1, 2, 4, 4, 8, 8, 8 — i.e. the requested size rounded up to a power of two.
The context ring buffer is allocated with capacity.next_power_of_two() slots and then retains
up to that many lines, rather than capping retention at the requested count.