git-gui: Delay searching for 'nice' until its really asked for

git-gui: Delay searching for 'nice' until its really asked for

Not every caller of 'git' or 'git_pipe' wants to use nice to lower the
priority of the process its executing.  In many cases we may never use
the nice process to launch git.  So we can avoid searching our $PATH
to locate a suitable nice if we'll never actually use it.

Signed-off-by: Shawn O. Pearce <redacted>