rust-regex: increase the DFA size limit for the `regex` crate

`re2`'s DFA limit is already increased in `rust/hg-core/src/re2/rust_re2.cpp`,
the same has to be done for the `regex` crate.

Big repositories with big `.hgignore`s will sometimes hit this limit and face
extreme performance regressions (I've seen one take *minutes* for `hg status`).

Differential Revision: https://phab.mercurial-scm.org/D8499

rust/hg-core/src/matchers.rs

@@ -358,6 +358,10 b' fn re_matcher('
     let pattern_string = unsafe { String::from_utf8_unchecked(escaped_bytes) };
     let re = regex::bytes::RegexBuilder::new(&pattern_string)
         .unicode(false)
+        // Big repos with big `.hgignore` will hit the default limit and
+        // incur a significant performance hit. One repo's `hg status` hit
+        // multiple *minutes*.
+        .dfa_size_limit(50 * (1 << 20))
         .build()
         .map_err(|e| PatternError::UnsupportedSyntax(e.to_string()))?;