pretty: fix memory leaks when parsing pretty formats

pretty: fix memory leaks when parsing pretty formats

When parsing pretty formats from the config we leak the name and user
format whenever these are set multiple times. This is because we do not
free any already-set value in case there is one.

Plugging this leak for the name is trivial. For the user format we need
to be a bit more careful, because we may end up assigning a pointer into
the allocated region when the string is prefixed with either "format" or
"tformat:". In order to make it safe to unconditionally free the user
format we thus strdup the stripped string into the field instead of a
pointer into the string.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>