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

--- a/mercurial/context.py	Tue Aug 14 22:20:28 2018 +0900
+++ b/mercurial/context.py	Wed Aug 15 17:40:21 2018 -0700
@@ -2033,6 +2033,13 @@
         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,