GNU start/end-of-buffer anchors are treated as literals instead of anchors

[sylvestre]

GNU grep supports two buffer anchors: \` matches the start of the buffer and \' matches the end (per line, in grep's line-oriented model these behave like ^/$). uu_grep does not recognise them — it treats \` and \' as the literal characters ` and ', so a pattern like t\' matches nothing instead of a line ending in t.

Found by the differential fuzzer (fuzz_grep).

Rust (incorrect)

$ printf 'cat\ndog\n' | ./target/release/grep -e "t\\'"
# Output: (none)
# Exit code: 1

GNU (correct)

$ printf 'cat\ndog\n' | LC_ALL=C /usr/bin/grep -e "t\\'"
cat
# Exit code: 0

The start-of-buffer anchor behaves the same way: \`c matches cat in GNU but nothing in uu_grep.