util._matcher: speed up regexp matching.

In 4babaa52badf, Benoit made a change that substantially slows matching
when a big .hgignore file is in play, because it calls into the regexp
matching engine potentially hundreds of times per file to be matched.

I've partly rolled back his change, so that we only call into the matcher
once per file, but preserved the ability to report a meaningful error
message if there's a syntax error in the regexp.

mercurial/util.py

@@ -469,22 +469,20 def _matcher(canonroot, cwd, names, inc,
         """build a matching function from a set of patterns"""
         if not pats:
             return
-        matches = []
+        try:
+            pat = '(?:%s)' % '|'.join([regex(k, p, tail) for (k, p) in pats])
+            return re.compile(pat).match
+        except re.error:
         for k, p in pats:
             try:
-                pat = '(?:%s)' % regex(k, p, tail)
-                matches.append(re.compile(pat).match)
+                    re.compile('(?:%s)' % regex(k, p, tail))
             except re.error:
-                if src: raise Abort("%s: invalid pattern (%s): %s" % (src, k, p))
-                else: raise Abort("invalid pattern (%s): %s" % (k, p))
-
-        def buildfn(text):
-            for m in matches:
-                r = m(text)
-                if r:
-                    return r
-
-        return buildfn
+                    if src:
+                        raise Abort("%s: invalid pattern (%s): %s" %
+                                    (src, k, p))
+                    else:
+                        raise Abort("invalid pattern (%s): %s" % (k, p))
+            raise Abort("invalid pattern")
 
     def globprefix(pat):
         '''return the non-glob prefix of a path, e.g. foo/* -> foo'''