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
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