CodingGuidelines: do not use 'which' in shell scripts

CodingGuidelines: do not use 'which' in shell scripts

During the code review of a recent patch, it was noted that shell scripts
must not use 'which $cmd' to check the availability of the command $cmd.
The output of the command is not machine parseable and its exit code is
not reliable across platforms.

It is better to use 'type' to accomplish this task.

Signed-off-by: Tim Henigan <tim.henigan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>