rust-regex: fix issues with regex anchoring and performance

It turns out that the way I tried to work around `regex`'s behavior difference
with `re2` and Python's `re` was 1) buggy and 2) much more complicated than
needed.

In a few words:
`regex` adds `.*` on either side of patterns when no start or end anchor is
present. My previous workaround put `^` or `$` for every pattern, which is
wrong even without the other 2 bugs on top of it.
Using `^(?:<patterns>)` right at the end of the `regex` path fixes the issue.
I've opened an issue to get a build option instead:
https://github.com/rust-lang/regex/issues/675

Differential Revision: https://phab.mercurial-scm.org/D8506

rust/hg-core/src/filepatterns.rs

@@ -176,9 +176,7 b' fn _build_single_regex(entry: &IgnorePat'
         return vec![];
     }
     match syntax {
-        // The `regex` crate adds `.*` to the start and end of expressions
-        // if there are no anchors, so add them.
-        PatternSyntax::Regexp => [b"^", &pattern[..], b"$"].concat(),
+        PatternSyntax::Regexp => pattern.to_owned(),
         PatternSyntax::RelRegexp => {
             // The `regex` crate accepts `**` while `re2` and Python's `re`
             // do not. Checking for `*` correctly triggers the same error all
@@ -196,15 +194,14 b' fn _build_single_regex(entry: &IgnorePat'
         }
         PatternSyntax::RootFiles => {
             let mut res = if pattern == b"." {
-                vec![b'^']
+                vec![]
             } else {
                 // Pattern is a directory name.
-                [b"^", escape_pattern(pattern).as_slice(), b"/"].concat()
+                [escape_pattern(pattern).as_slice(), b"/"].concat()
             };
 
             // Anything after the pattern must be a non-directory.
             res.extend(b"[^/]+$");
-            res.push(b'$');
             res
         }
         PatternSyntax::RelGlob => {
@@ -216,7 +213,7 b' fn _build_single_regex(entry: &IgnorePat'
             }
         }
         PatternSyntax::Glob | PatternSyntax::RootGlob => {
-            [b"^", glob_to_re(pattern).as_slice(), GLOB_SUFFIX].concat()
+            [glob_to_re(pattern).as_slice(), GLOB_SUFFIX].concat()
         }
         PatternSyntax::Include | PatternSyntax::SubInclude => unreachable!(),
     }

rust/hg-core/src/matchers.rs

@@ -347,7 +347,9 b' fn re_matcher('
 ) -> PatternResult<impl Fn(&HgPath) -> bool + Sync> {
     use std::io::Write;
 
-    let mut escaped_bytes = vec![];
+    // The `regex` crate adds `.*` to the start and end of expressions if there
+    // are no anchors, so add the start anchor.
+    let mut escaped_bytes = vec![b'^', b'(', b'?', b':'];
     for byte in pattern {
         if *byte > 127 {
             write!(escaped_bytes, "\\x{:x}", *byte).unwrap();
@@ -355,6 +357,7 b' fn re_matcher('
             escaped_bytes.push(*byte);
         }
     }
+    escaped_bytes.push(b')');
 
     // Avoid the cost of UTF8 checking
     //