lazymanifest: avoid reading uninitialized memory stable

lazymanifest: avoid reading uninitialized memory

I got errors running tests with clang UBSAN [1] enabled. One of them is:

```
--- test-dirstate.t
+++ test-dirstate.t.err
@@ -85,9 +85,115 @@
   $ echo "[extensions]" >> .hg/hgrc
   $ echo "dirstateex=../dirstateexception.py" >> .hg/hgrc
   $ hg up 0
-  abort: simulated error while recording dirstateupdates
-  [255]
+  mercurial/cext/manifest.c:781:13: runtime error: load of value 190, which is not a valid value for type 'bool'
+      #0 0x7f668a8cf748 in lazymanifest_diff mercurial/cext/manifest.c:781
+      #1 0x7f6692fc1dc4 in call_function Python-2.7.11/Python/ceval.c:4350
+      .......
+  SUMMARY: UndefinedBehaviorSanitizer: invalid-bool-load mercurial/cext/manifest.c:781:13 in
+  [1]
   $ hg log -r . -T '{rev}\n'
   1
   $ hg status
-  ? a
```

While the code is not technically wrong, but switching the condition would
make clang UBSAN happy. So let's do it.

The uninitialized memory could come from, for example, `lazymanifest_copy`
allocates `self->maxlines` items but only writes the first `self->lines`
items.

[1]: https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html

Test Plan:
Run `test-dirstate.t` with UBSAN and it no longer reports the issue.

Differential Revision: https://phab.mercurial-scm.org/D1948

--- a/mercurial/cext/manifest.c	Fri Jan 26 11:42:47 2018 -0800
+++ b/mercurial/cext/manifest.c	Tue Jan 30 20:32:48 2018 -0800
@@ -778,11 +778,11 @@
 		PyObject *outer;
 		/* If we're looking at a deleted entry and it's not
 		 * the end of the manifest, just skip it. */
-		if (left->deleted && sneedle < self->numlines) {
+		if (sneedle < self->numlines && left->deleted) {
 			sneedle++;
 			continue;
 		}
-		if (right->deleted && oneedle < other->numlines) {
+		if (oneedle < other->numlines && right->deleted) {
 			oneedle++;
 			continue;
 		}