git-p4: don't print shell commands as python lists

git-p4: don't print shell commands as python lists

Previously the git-p4 script would log commands as stringified
representations of the command parameter, leading to output such as
this:

Reading pipe: ['git', 'config', '--bool', 'git-p4.useclientspec']

Now that all commands are list objects, this patch instead joins the
elements of the list into a single string so the output now looks more
readable:

Reading pipe: git config --bool git-p4.useclientspec

Signed-off-by: Joel Holdsworth <jholdsworth@nvidia.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>