Documentation: avoid poor-man's small caps GIT

Documentation: avoid poor-man's small caps GIT

In the earlier days, we used to spell the name of the system as GIT,
to simulate as if it were typeset with capital G and IT in small
caps.  Later we stopped doing so at around 1.6.5 days.

Let's stop doing so throughout the documentation.  The name to refer
to the whole system (and the concept it embodies) is "Git"; the
command end-users type is "git".  And document this in the coding
guideline.

Signed-off-by: Thomas Ackermann <th.acker@arcor.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>