patch: add fuzz config flag (issue4697)

mercurial/help/config.txt

@@ -1093,6 +1093,11 command or with Mercurial Queues extensi
     of line, patch line endings are preserved.
     Default: strict.
 
+``fuzz``
+    The number of lines of 'fuzz' to allow when applying patches. This
+    controls how much context the patcher is allowed to ignore when
+    trying to apply a patch.
+    Default: 2
 
 ``paths``
 ---------

mercurial/patch.py

@@ -773,7 +773,7 class patchfile(object):
         for x, s in enumerate(self.lines):
             self.hash.setdefault(s, []).append(x)
 
-        for fuzzlen in xrange(3):
+        for fuzzlen in xrange(self.ui.configint("patch", "fuzz", 2) + 1):
             for toponly in [True, False]:
                 old, oldstart, new, newstart = h.fuzzit(fuzzlen, toponly)
                 oldstart = oldstart + self.offset + self.skew

tests/test-import.t

@@ -492,6 +492,13 Test fuzziness (ambiguous patch location
   $ echo line0 >> a
   $ hg ci -m brancha
   created new head
+  $ hg import --config patch.fuzz=0 -v fuzzy-tip.patch
+  applying fuzzy-tip.patch
+  patching file a
+  Hunk #1 FAILED at 0
+  1 out of 1 hunks FAILED -- saving rejects to file a.rej
+  abort: patch failed to apply
+  [255]
   $ hg import --no-commit -v fuzzy-tip.patch
   applying fuzzy-tip.patch
   patching file a