p4 unshelve: fix "Not a valid object name HEAD0" on Windows

p4 unshelve: fix "Not a valid object name HEAD0" on Windows

git p4 unshelve was failing with these errors:

fatal: Not a valid object name HEAD0
Command failed: git cat-file commit HEAD^0

(git version 2.21.0.windows.1, python 2.7.16)

The pOpen call used by git-p4 to invoke the git command can take either a
string or an array as a first argument. The array form is preferred
because platform-specific escaping of special characters will be
handled automatically.(https://docs.python.org/2/library/subprocess.html)
The extractLogMessageFromGitCommit method was, however, using the string
form and so the caret (^) character in the HEAD^0 argument was not being
escaped on Windows.  The caret happens to be the escape character, which
is why the git command was receiving HEAD0.

The behaviour can be confirmed by typing ECHO HEAD^0 at the command-
prompt, which emits HEAD0.

The solution is simply to use the array format of passing the command to
fOpen, which is recommended and used in other parts of this code anyway.

Signed-off-by: Mike Mueller <mike.mueller@moodys.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>