searcher: bump buffer size

This increases the initial buffer size from 8KB to 64KB. This actually
leads to a reasonably noticeable improvement in at least one work-load,
and is unlikely to regress in any other case. Also, since Rust programs
(at least on Linux) seem to always use a minimum of 6-8MB of memory,
adding an extra 56KB is negligible.

Before:

    $ hyperfine -i "rg 'zqzqzqzq' OpenSubtitles2018.raw.en --no-mmap"
    Benchmark #1: rg 'zqzqzqzq' OpenSubtitles2018.raw.en --no-mmap
      Time (mean ± σ):      2.109 s ±  0.012 s    [User: 565.5 ms, System: 1541.6 ms]
      Range (min … max):    2.094 s …  2.128 s    10 runs

After:

    $ hyperfine -i "rg 'zqzqzqzq' OpenSubtitles2018.raw.en --no-mmap"
    Benchmark #1: rg 'zqzqzqzq' OpenSubtitles2018.raw.en --no-mmap
      Time (mean ± σ):      1.802 s ±  0.006 s    [User: 462.3 ms, System: 1337.9 ms]
      Range (min … max):    1.795 s …  1.814 s    10 runs

Author: BurntSushi

crates/searcher/src/line_buffer.rs

@@ -4,7 +4,7 @@ use std::io;
 use bstr::ByteSlice;
 
 /// The default buffer capacity that we use for the line buffer.
-pub(crate) const DEFAULT_BUFFER_CAPACITY: usize = 8 * (1 << 10); // 8 KB
+pub(crate) const DEFAULT_BUFFER_CAPACITY: usize = 64 * (1 << 10); // 8 KB
 
 /// The behavior of a searcher in the face of long lines and big contexts.
 ///