Documentation: make environment variable formatting more consistent

Documentation: make environment variable formatting more consistent

Documentation/git.txt is not consistent in the way it
stylizes mentions of Environment Variables. Most of them are
enclosed in single quotes, some are enclosed in backticks,
some are not enclosed.

Signed-off-by: Andrey Rybak <rybak.a.v@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>