Send stderr to subprocess.PIPE in version.py

If git is not installed, the stderr output from the shell out to 'which
git' is printed to the console whenever a salt CLI command is run. This
commit sends it to the subprocess pipe to keep it away from the console.

Also, I replaced the call to 'which git' with the POSIX-compliant
'command -v git'. The 'which' command should be avoided as it is not
POSIX-compliant, and doesn't set a nonzero return code on all platforms.
See below for an example of how which does not work as expected on
Solaris. I believe the MacOS version of which also has this problem. On
these platforms, if git was not found, the call to
subprocess.Popen.poll() would not find a nonzero exit status and salt
would therefore continue trying to obtain the current git commit ID.

$ uname -a
SunOS foo.tld 5.10 Generic_139556-08 i86pc i386 i86pc
$ which foo
no foo in /usr/pkg/bin /usr/pkg/sbin /usr/sfw/bin /usr/sfw/sbin /sbin
/usr/sbin /usr/ccs/bin /usr/bin
$ echo $?
0
$ command -v foo
-bash: command: foo: not found
$ echo $?
1
Merge pull request #3461 from archtaku/quickfix

Send stderr to subprocess.PIPE in version.py