[PATCH 0 of 4] Begin adding tests and small fixes for Largefiles Extension

[Na'Tosha Bard]

NOTE: This replaces the previous patchbomb.

This patchbomb starts the process of adding tests (and fixes for things I find as I write the tests) for the largefiles extension.  These tests cover basic functionality of mercurial operations on largefiles, as well as some more obscure functionality that I know to have been very problematic (hard to fix and easy to break) for largefiles in the past.