p4fastimport: better handling of symlinks that later turn in to directories
We don't actually need to store fileflags='', which uses a lot of memory
because there are many files. Instead, just treat missing fileflags as empty.
Also, no need to call manifest.setflag() for every file -- call
manifest.flags() for every file, then setflag() only if needed.
Differential Revision: https://phab.mercurial-scm.org/D1315