patch: allow copy information to be passed in

When displaying patches from graphical tools where you can browse through
individual files, with diff being called separately on each, recomputing the
limits of file copy history can become rather expensive on large repositories.
Instead, we can compute it once and pass it in for subsequent calls.

mercurial/patch.py

@@ -2184,7 +2184,7 b' def difffeatureopts(ui, opts=None, untru'
     return mdiff.diffopts(**buildopts)
 
 def diff(repo, node1=None, node2=None, match=None, changes=None, opts=None,
-         losedatafn=None, prefix='', relroot=''):
+         losedatafn=None, prefix='', relroot='', copy=None):
     '''yields diff of changes to files between two nodes, or node and
     working directory.
 
@@ -2203,7 +2203,10 b' def diff(repo, node1=None, node2=None, m'
     display (used for subrepos).
 
     relroot, if not empty, must be normalized with a trailing /. Any match
-    patterns that fall outside it will be ignored.'''
+    patterns that fall outside it will be ignored.
+
+    copy, if not empty, should contain mappings {dst@y: src@x} of copy
+    information.'''
 
     if opts is None:
         opts = mdiff.defaultopts
@@ -2250,9 +2253,10 b' def diff(repo, node1=None, node2=None, m'
         hexfunc = short
     revs = [hexfunc(node) for node in [ctx1.node(), ctx2.node()] if node]
 
-    copy = {}
-    if opts.git or opts.upgrade:
-        copy = copies.pathcopies(ctx1, ctx2, match=match)
+    if copy is None:
+        copy = {}
+        if opts.git or opts.upgrade:
+            copy = copies.pathcopies(ctx1, ctx2, match=match)
 
     if relroot is not None:
         if not relfiltered: