Disable git's pager.

Inspired by #415.

While I haven't reproduced the problem, I can imagine that git's pager could cause spurious error codes. `CommandOneLine` seems especially likely to trigger the error, as it may close the pager's stdout before it has a chance to do anything.

This (should) disable git's pager for everything that we still use `Process.Start` for. (Long-term, of course, the plan is to :fire: `GitHelpers` :fire:.)

cc @drewnoakes