Change mentions of "git programs" to "git commands"

Change mentions of "git programs" to "git commands"

Most of the docs and printouts refer to "commands" when discussing what
the end users call via the "git" top-level program. We should refer them
as "git programs" when we discuss the fact that the commands are
implemented as separate programs, but in other contexts, it is better to
use the term "git commands" consistently.

Signed-off-by: Ori Avtalion <ori@avtalion.name>
Signed-off-by: Nanako Shiraishi <nanako3@lavabit.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>