overlayworkingctx: fix exception in metadata-only inmemory merges (issue5960)

If there was a metadata-only mutation, such as +x or -x on a file, we would
create a cache entry with None for data, and this would cause problems later on
when some code tried to run fctx.data() or similar, and was expecting a string.

My original fix for this involved passing data=self._wrappedctx[path].data() in
setflags(), but this version seems slightly better - this way, if we ever call
write() and then call setflags(), we don't destroy the data that we wrote that's
in the cache. I haven't verified that other fields aren't destroyed, such as
date or flags :)

Differential Revision: https://phab.mercurial-scm.org/D4287

mercurial/context.py

@@ -2033,6 +2033,13 b' class overlayworkingctx(committablectx):'
         return keys
 
     def _markdirty(self, path, exists, data=None, date=None, flags=''):
+        # data not provided, let's see if we already have some; if not, let's
+        # grab it from our underlying context, so that we always have data if
+        # the file is marked as existing.
+        if exists and data is None:
+            oldentry = self._cache.get(path) or {}
+            data = oldentry.get('data') or self._wrappedctx[path].data()
+
         self._cache[path] = {
             'exists': exists,
             'data': data,

tests/test-rebase-inmemory.t

@@ -509,3 +509,31 b' Test --confirm option when there is a co'
   o  0:cb9a9f314b8b test
      a
   
+#if execbit
+
+Test a metadata-only in-memory merge
+  $ cd $TESTTMP
+  $ hg init no_exception
+  $ cd no_exception
+# Produce the following graph:
+#   o  'add +x to foo.txt'
+#   | o  r1  (adds bar.txt, just for something to rebase to)
+#   |/
+#   o  r0   (adds foo.txt, no +x)
+  $ echo hi > foo.txt
+  $ hg ci -qAm r0
+  $ echo hi > bar.txt
+  $ hg ci -qAm r1
+  $ hg co -qr ".^"
+  $ chmod +x foo.txt
+  $ hg ci -qAm 'add +x to foo.txt'
+issue5960: this was raising an AttributeError exception
+  $ hg rebase -r . -d 1
+  rebasing 2:539b93e77479 "add +x to foo.txt" (tip)
+  saved backup bundle to $TESTTMP/no_exception/.hg/strip-backup/*.hg (glob)
+  $ hg diff -c tip
+  diff --git a/foo.txt b/foo.txt
+  old mode 100644
+  new mode 100755
+
+#endif