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>

Documentation/CodingGuidelines

@@ -46,6 +46,11 @@ For shell scripts specifically (not exhaustive):
    properly nests.  It should have been the way Bourne spelled
    it from day one, but unfortunately isn't.
 
+ - If you want to find out if a command is available on the user's
+   $PATH, you should use 'type <command>', instead of 'which <command>'.
+   The output of 'which' is not machine parseable and its exit code
+   is not reliable across platforms.
+
  - We use POSIX compliant parameter substitutions and avoid bashisms;
    namely: