[PATCH 0 of 3] Provide *_ISLINK environment vars to merge helper

[Patrick Mezard]

The new .fileflags() version avoids the traceback when called on a fresh new workingctx without _manifest (while .filenode() still suffers from it), and should be much faster since it avoir rebuilding parent rev manifest.

It could be even faster if instead of calling self.deleted() and self.removed(), we were calling status(files=[path]). I do not know if this is that useful, tell me.