crecord: add `content` properties to all nodes

In order to have a unified API of what can be searched, let's provide
a `content` property to each node type. This way we can search
filenames, context headers (e.g. containing function names, if
deducible from patch context) or changed lines themselves.

mercurial/crecord.py

@@ -86,6 +86,10 b' class patchnode:'
     (i.e. patchroot, header, hunk, hunkline)
     """
 
+    @property
+    def content(self):
+        return b''
+
     def firstchild(self):
         raise NotImplementedError(b"method must be implemented by subclass")
 
@@ -223,6 +227,10 b' class uiheader(patchnode):'
         self.neverunfolded = True
         self.hunks = [uihunk(h, self) for h in self.hunks]
 
+    @property
+    def content(self):
+        return self.filename()
+
     def prettystr(self):
         x = stringio()
         self.pretty(x)
@@ -289,6 +297,10 b' class uihunkline(patchnode):'
         # in the previtem method.
         self.folded = False
 
+    @property
+    def content(self):
+        return self.linetext
+
     def prettystr(self):
         return self.linetext
 
@@ -347,6 +359,10 b' class uihunk(patchnode):'
         # children are partially applied (i.e. some applied, some not).
         self.partial = False
 
+    @property
+    def content(self):
+        return self.proc if self.proc else b''
+
     def nextsibling(self):
         numhunksinheader = len(self.header.hunks)
         indexofthishunk = self.header.hunks.index(self)