patch: fix fuzzing of hunks without previous lines (issue3264)

When applying hunks such as:

  @@ -2,1 +2,2 @@
   context
  +change

fuzzing would empty the "old" block and make patchfile.apply() traceback.
Instead, we apply the new block at specified location without testing.

The "bottom hunk" test was removed as patch(1) has no problem applying hunk
with no context in the middle of a file.

mercurial/patch.py

@@ -748,20 +748,21 b' class patchfile(object):'
         self.hash = {}
         for x, s in enumerate(self.lines):
             self.hash.setdefault(s, []).append(x)
-        if h.hunk[-1][0] != ' ':
-            # if the hunk tried to put something at the bottom of the file
-            # override the start line and use eof here
-            search_start = len(self.lines)
-        else:
-            search_start = orig_start + self.skew
 
         for fuzzlen in xrange(3):
             for toponly in [True, False]:
                 old, oldstart, new, newstart = h.fuzzit(fuzzlen, toponly)
+                oldstart = oldstart + self.offset + self.skew
+                oldstart = min(oldstart, len(self.lines))
+                if old:
+                    cand = self.findlines(old[0][1:], oldstart)
+                else:
+                    # Only adding lines with no or fuzzed context, just
+                    # take the skew in account
+                    cand = [oldstart]
 
-                cand = self.findlines(old[0][1:], search_start)
                 for l in cand:
-                    if diffhelpers.testhunk(old, self.lines, l) == 0:
+                    if not old or diffhelpers.testhunk(old, self.lines, l) == 0:
                         self.lines[l : l + len(old)] = new
                         self.offset += len(new) - len(old)
                         self.skew = l - orig_start

tests/test-import.t

@@ -996,3 +996,79 b' import a unified diff with no lines of c'
   c2
   c3
   c4
+
+Test corner case involving fuzz and skew
+
+  $ hg init morecornercases
+  $ cd morecornercases
+
+  $ cat > 01-no-context-beginning-of-file.diff <<EOF
+  > diff --git a/a b/a
+  > --- a/a
+  > +++ b/a
+  > @@ -1,0 +1,1 @@
+  > +line
+  > EOF
+
+  $ cat > 02-no-context-middle-of-file.diff <<EOF
+  > diff --git a/a b/a
+  > --- a/a
+  > +++ b/a
+  > @@ -1,1 +1,1 @@
+  > -2
+  > +add some skew
+  > @@ -2,0 +2,1 @@
+  > +line
+  > EOF
+
+  $ cat > 03-no-context-end-of-file.diff <<EOF
+  > diff --git a/a b/a
+  > --- a/a
+  > +++ b/a
+  > @@ -10,0 +10,1 @@
+  > +line
+  > EOF
+
+  $ cat > a <<EOF
+  > 1
+  > 2
+  > 3
+  > 4
+  > EOF
+  $ hg ci -Am adda a
+  $ for p in *.diff; do
+  >   hg import -v --no-commit $p
+  >   cat a
+  >   hg revert -aqC a
+  >   # patch -p1 < $p
+  >   # cat a
+  >   # hg revert -aC a
+  > done
+  applying 01-no-context-beginning-of-file.diff
+  patching file a
+  applied to working directory
+  1
+  line
+  2
+  3
+  4
+  applying 02-no-context-middle-of-file.diff
+  patching file a
+  Hunk #1 succeeded at 2 (offset 1 lines).
+  Hunk #2 succeeded at 4 (offset 1 lines).
+  applied to working directory
+  1
+  add some skew
+  3
+  line
+  4
+  applying 03-no-context-end-of-file.diff
+  patching file a
+  Hunk #1 succeeded at 5 (offset -6 lines).
+  applied to working directory
+  1
+  2
+  3
+  4
+  line
+