patch: move yielding "\n" to the end of loop

The original logic makes it harder to reason about - it yields the "\n"
character belonging to the last line in the next loop iteration.

The new code is in theory a little bit slower. But is more readable. It
makes the following changes easier to read.

Differential Revision: https://phab.mercurial-scm.org/D3210

mercurial/patch.py

@@ -2505,9 +2505,8 b' def difflabel(func, *args, **kw):'
         matches = {}
         if inlinecolor:
             matches = _findmatches(lines)
+        linecount = len(lines)
         for i, line in enumerate(lines):
-            if i != 0:
-                yield ('\n', '')
             if head:
                 if line.startswith('@'):
                     head = False
@@ -2546,6 +2545,8 b' def difflabel(func, *args, **kw):'
                 yield (line, '')
             if line != stripline:
                 yield (line[len(stripline):], 'diff.trailingwhitespace')
+            if i + 1 < linecount:
+                yield ('\n', '')
 
 def _findmatches(slist):
     '''Look for insertion matches to deletion and returns a dict of