git-p4: change the expansion test from basestring to list

Python 3 handles strings differently than Python 2.7. Since Python 2 is reaching it's end of life, a series of changes are being submitted to enable python 3.5 and following support. The current code fails basic tests under python 3.5. Some codepaths can represent a command line the program internally prepares to execute either as a single string (i.e. each token properly quoted, concatenated with $IFS) or as a list of argv[] elements, and there are 9 places where we say "if X is isinstance(_, basestring), then do this thing to handle X as a command line in a single string; if not, X is a command line in a list form". This does not work well with Python 3, as there is no basestring (everything is Unicode now), and even with Python 2, it was not an ideal way to tell the two cases apart, because an internally formed command line could have been in a single Unicode string. Flip the check to say "if X is not a list, then handle X as a command line in a single string; otherwise treat it as a command line in a list form". This will get rid of references to 'basestring', to migrate the code ready for Python 3. Thanks-to: Junio C Hamano <gitster@pobox.com> Signed-off-by: Ben Keene <seraphire@gmail.com> Signed-off-by: Yang Zhao <yang.zhao@skyboxlabs.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>