Summary:
If an executable filenode was reused from p2 and mode hasn't been
changed (i.e. a filenode was executable and it is still an executable after a
merge) and and p1 didn't have this file at all then file will still be listed as
changed just because manifest1.flags(fname) returns ''.
After this diff if a filenode is reused then file will be listed as changed
only if it's a parent from where this filenode was reused have different flags.
So if filenode was reused from p1/p2 and p1/p2 has same flags and new
filenode, then file is not listed. But if filenode was reused from p1 and p1
has different flags, then filenode won't be reused.
It's possible that both new filenode is equal to both p1 and p2. In that case
we'll compare only with p1's mode.
Note that it's only get triggered during pushrebase, because during local
commits `fctx.filenode()` is None
Reviewed By: DurhamG
Differential Revision: D14300214
fbshipit-source-id: 1bf6c4802cfce5db6654da673333a56389432617
