diff: fix obscure off-by-one error in diff -p

mercurial/mdiff.py

@@ -160,7 +160,7 b' def bunidiff(t1, t2, l1, l2, header1, he'
         if opts.showfunc:
             # walk backwards from the start of the context
             # to find a line starting with an alphanumeric char.
-            for x in xrange(astart, -1, -1):
+            for x in xrange(astart - 1, -1, -1):
                 t = l1[x].rstrip()
                 if funcre.match(t):
                     func = ' ' + t[:40]

tests/test-diff-unified

@@ -44,6 +44,15 b' hg diff --nodates -U 1'
 echo '% invalid diff.unified'
 hg --config diff.unified=foo diff --nodates
 
-exit 0
+echo % test off-by-one error with diff -p
+hg init diffp
+cd diffp
+echo a > a
+hg ci -Ama
+rm a
+echo b > a
+echo a >> a
+echo c >> a
+hg diff -U0 -p
 
-
+exit 0