regex: fix a perf bug when using -w flag

When looking for an inner literal to speed up searches, if only a prefix
is found, then we generally give up doing inner literal optimizations since
the regex engine will generally handle it for us. Unfortunately, this
decision was being made *before* we wrap the regex in (^|\W)...($|\W) when
using the -w/--word-regexp flag, which would then defeat the literal
optimizations inside the regex engine.

We fix this with a bit of a hack that says, "if we're doing a word regexp,
then give me back any literal you find, even if it's a prefix."

grep-regex/src/config.rs

@@ -207,7 +207,7 @@ impl ConfiguredHIR {
         if self.config.line_terminator.is_none() {
             return Ok(None);
         }
-        match LiteralSets::new(&self.expr).one_regex() {
+        match LiteralSets::new(&self.expr).one_regex(self.config.word) {
             None => Ok(None),
             Some(pattern) => self.pattern_to_regex(&pattern).map(Some),
         }