git-p4: add config to retry p4 commands; retry 3 times by default

git-p4: add config to retry p4 commands; retry 3 times by default

P4 commands can fail due to random network issues. P4 users can counter
these issues by using a retry flag supported by all p4 commands [1].

Add an integer Git config value `git-p4.retries` to define the number of
retries for all p4 invocations. If the config is not defined then set
the default retry count to 3.

[1] https://www.perforce.com/perforce/doc.current/manuals/cmdref/global.options.html

Signed-off-by: Lars Schneider <larsxschneider@gmail.com>
Reviewed-by: Luke Diamand <luke@diamand.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>