assigned missing remote callback
init helper for CheckoutOptions
added optional initialization and comparison operators for StrArray
an empty QStringList must assign NULL to the strings in the converted git_strarray
fixed initialization of Internal::CheckoutOptions

As git_init_options() reassigns the pointer to the paths array,
the initialization must be done afterwards.
Otherwise the array produces a memory leak,
when the QStringList is assigned before.

The asserts assure, that the initialization "under the hood"
does not point to another instance in the future.
Attention, API change! DiffList should behave GitWrap conform.