git-p4: handle files with shell metacharacters

git-p4 used to simply pass strings into system() and popen(), and
relied on the shell doing the necessary expansion. This though meant
that shell metacharacters in file names would be corrupted - for
example files with $ or space in them.

Switch to using subprocess.Popen() and friends, and pass in explicit
arrays in the places where it matters. This then avoids needing shell
expansion.

Add trivial helper functions for some common perforce operations. Add
test case.

[pw: test cleanup]

Signed-off-by: Luke Diamand <luke@diamand.org>
Signed-off-by: Pete Wyckoff <pw@padd.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>