diff: move diffline to patch module

diff: move diffline to patch module

diffline is not part of diff computation, so it makes more sense
to place it with other header generation in patch module.

In upcoming patches we will generalize this approach for
all headers added in the patch, including the git index
header.

--- a/mercurial/mdiff.py	Thu Nov 15 12:16:08 2012 -0800
+++ b/mercurial/mdiff.py	Thu Nov 15 12:19:03 2012 -0800
@@ -143,19 +143,6 @@
             yield s, type
         yield s1, '='
 
-def diffline(revs, a, b, opts):
-    parts = ['diff']
-    if opts.git:
-        parts.append('--git')
-    if revs and not opts.git:
-        parts.append(' '.join(["-r %s" % rev for rev in revs]))
-    if opts.git:
-        parts.append('a/%s' % a)
-        parts.append('b/%s' % b)
-    else:
-        parts.append(a)
-    return ' '.join(parts) + '\n'
-
 def unidiff(a, ad, b, bd, fn1, fn2, opts=defaultopts):
     def datetag(date, fn=None):
         if not opts.git and not opts.nodates:

--- a/mercurial/patch.py	Thu Nov 15 12:16:08 2012 -0800
+++ b/mercurial/patch.py	Thu Nov 15 12:19:03 2012 -0800
@@ -1664,6 +1664,19 @@
     def join(f):
         return os.path.join(prefix, f)
 
+    def diffline(revs, a, b, opts):
+        parts = ['diff']
+        if opts.git:
+            parts.append('--git')
+        if revs and not opts.git:
+            parts.append(' '.join(["-r %s" % rev for rev in revs]))
+        if opts.git:
+            parts.append('a/%s' % a)
+            parts.append('b/%s' % b)
+        else:
+            parts.append(a)
+        return ' '.join(parts) + '\n'
+
     date1 = util.datestr(ctx1.date())
     man1 = ctx1.manifest()
 
@@ -1749,7 +1762,7 @@
 
         if dodiff:
             if opts.git or revs:
-                header.insert(0, mdiff.diffline(revs, join(a), join(b), opts))
+                header.insert(0, diffline(revs, join(a), join(b), opts))
             if dodiff == 'binary':
                 text = mdiff.b85diff(to, tn)
             else: