revset: add "diff" field to "matching" keyword

The new "diff" field lets you use the matching revset keyword to find revisions
that apply the same change as the selected revisions.

The match must be exact (i.e. same additions, same deletions, same modified
lines and same change context, same file renames and copies).

Two revisions matching their diff must also match their files. Thus, to match
the diff much faster we will always check that the 'files' match first, and only
then check that the 'diff' matches as well.

mercurial/revset.py

@@ -1058,8 +1058,11 b' def matching(repo, subset, x):'
     Valid fields are most regular revision fields and some special fields.
 
     Regular revision fields are ``description``, ``author``, ``branch``,
-    ``date``, ``files``, ``phase``, ``parents``, ``substate`` and ``user``.
-    Note that ``author`` and ``user`` are synonyms.
+    ``date``, ``files``, ``phase``, ``parents``, ``substate``, ``user``
+    and ``diff``.
+    Note that ``author`` and ``user`` are synonyms. ``diff`` refers to the
+    contents of the revision. Two revisions matching their ``diff`` will
+    also match their ``files``.
 
     Special fields are ``summary`` and ``metadata``:
     ``summary`` matches the first line of the description.
@@ -1079,12 +1082,18 b' def matching(repo, subset, x):'
                 _("matching requires a string "
                 "as its second argument")).split()
 
-    # Make sure that there are no repeated fields, and expand the
-    # 'special' 'metadata' field type
+    # Make sure that there are no repeated fields,
+    # expand the 'special' 'metadata' field type
+    # and check the 'files' whenever we check the 'diff'
     fields = []
     for field in fieldlist:
         if field == 'metadata':
             fields += ['user', 'description', 'date']
+        elif field == 'diff':
+            # a revision matching the diff must also match the files
+            # since matching the diff is very costly, make sure to
+            # also match the files first
+            fields += ['files', 'diff']
         else:
             if field == 'author':
                 field = 'user'
@@ -1098,7 +1107,7 b' def matching(repo, subset, x):'
     # Not all fields take the same amount of time to be matched
     # Sort the selected fields in order of increasing matching cost
     fieldorder = ['phase', 'parents', 'user', 'date', 'branch', 'summary',
-        'files', 'description', 'substate']
+        'files', 'description', 'substate', 'diff']
     def fieldkeyfunc(f):
         try:
             return fieldorder.index(f)
@@ -1121,6 +1130,7 b' def matching(repo, subset, x):'
         'phase': lambda r: repo[r].phase(),
         'substate': lambda r: repo[r].substate,
         'summary': lambda r: repo[r].description().splitlines()[0],
+        'diff': lambda r: list(repo[r].diff(git=True),)
     }
     for info in fields:
         getfield = _funcs.get(info, None)