largefiles: make update with backup files in .hglf slightly less broken
Largefiles update would try to copy f to f.orig if there was a .hglf/f.orig .
That is in many many ways very very wrong, but it also caused an abort if f
didn't exist.
Now it only tries to copy f if it exists.
.. Common link and substitution definitions.
.. |hg(1)| replace:: **hg**\ (1)
.. _hg(1): hg.1.html
.. |hgrc(5)| replace:: **hgrc**\ (5)
.. _hgrc(5): hgrc.5.html
.. |hgignore(5)| replace:: **hgignore**\ (5)
.. _hgignore(5): hgignore.5.html