add debugignore which yields the combined ignore patten of the .hgignore files

For GUI clients its sometimes important to know which files will be ignored and
which files will be important. This allows the GUI client to skipping redoing a
'hg status' when the files are ignored but have changed. (For instance, a
typical case is that the "build" directory inside some project is ignored but
files in it frequently change.)

mercurial/commands.py

@@ -1351,6 +1351,11 b' def debugdate(ui, date, range=None, **op'
         m = util.matchdate(range)
         ui.write("match: %s\n" % m(d[0]))
 
+def debugignore(ui, repo, *values, **opts):
+    """display the combined ignore pattern"""
+    ignore = repo.dirstate._ignore
+    ui.write("%s\n" % ignore.includepat)
+
 def debugindex(ui, repo, file_, **opts):
     """dump the contents of an index file"""
     r = None
@@ -4347,6 +4352,7 b' table = {'
          _('[-e] DATE [RANGE]')),
     "debugdata": (debugdata, [], _('FILE REV')),
     "debugfsinfo": (debugfsinfo, [], _('[PATH]')),
+    "debugignore": (debugignore, [], ''),
     "debugindex": (debugindex,
                    [('f', 'format', 0, _('revlog format'), _('FORMAT'))],
                    _('FILE')),

mercurial/match.py

@@ -39,11 +39,11 b' class match(object):'
         self._anypats = bool(include or exclude)
 
         if include:
-            im = _buildmatch(_normalize(include, 'glob', root, cwd, auditor),
-                             '(?:/|$)')
+            pats = _normalize(include, 'glob', root, cwd, auditor)
+            self.includepat, im = _buildmatch(pats, '(?:/|$)')
         if exclude:
-            em = _buildmatch(_normalize(exclude, 'glob', root, cwd, auditor),
-                             '(?:/|$)')
+            pats = _normalize(exclude, 'glob', root, cwd, auditor)
+            self.excludepat, em = _buildmatch(pats, '(?:/|$)')
         if exact:
             self._files = patterns
             pm = self.exact
@@ -51,7 +51,7 b' class match(object):'
             pats = _normalize(patterns, default, root, cwd, auditor)
             self._files = _roots(pats)
             self._anypats = self._anypats or _anypats(pats)
-            pm = _buildmatch(pats, '$')
+            self.patternspat, pm = _buildmatch(pats, '$')
 
         if patterns or exact:
             if include:
@@ -246,7 +246,7 b' def _buildmatch(pats, tail):'
         pat = '(?:%s)' % '|'.join([_regex(k, p, tail) for (k, p) in pats])
         if len(pat) > 20000:
             raise OverflowError()
-        return re.compile(pat).match
+        return pat, re.compile(pat).match
     except OverflowError:
         # We're using a Python with a tiny regex engine and we
         # made it explode, so we'll divide the pattern list in two
@@ -254,8 +254,9 b' def _buildmatch(pats, tail):'
         l = len(pats)
         if l < 2:
             raise
-        a, b = _buildmatch(pats[:l//2], tail), _buildmatch(pats[l//2:], tail)
-        return lambda s: a(s) or b(s)
+        pata, a = _buildmatch(pats[:l//2], tail),
+        patb, b = _buildmatch(pats[l//2:], tail)
+        return pat, lambda s: a(s) or b(s)
     except re.error:
         for k, p in pats:
             try:

tests/test-debugcomplete.t

@@ -75,6 +75,7 b' Show debug commands if there are no othe'
   debugdata
   debugdate
   debugfsinfo
+  debugignore
   debugindex
   debugindexdot
   debuginstall
@@ -214,6 +215,7 b' Show all commands + options'
   debugdata: 
   debugdate: extended
   debugfsinfo: 
+  debugignore: 
   debugindex: format
   debugindexdot: 
   debuginstall: 

tests/test-hgignore.t

@@ -120,3 +120,5 b' Check it does not ignore the current dir'
   $ hg status .
   A b.o
 
+  $ hg debugignore
+  (?:(?:|.*/)[^/]*(?:/|$))