git.c: Rename NEEDS_PREFIX to RUN_SETUP

git.c: Rename NEEDS_PREFIX to RUN_SETUP

As Matthias Kestenholz noted, the flag does not quite mean
"needs prefix" -- it is more like "run setup_git_directory()
before running this command", so rename it to avoid future
confusion.

While we are at it, rewrite the definition of options to make it
obvious that we are talking about flag bits by using standard (1<<n)
notation.

Signed-off-by: Junio C Hamano <junkio@cox.net>