This was not converted when we converted the rest, so do it now.

This was left over when we did the general switch.

Okay, I've decided I like the readability of this style much
better so I used it everywhere.

This changes the behavior of callbacks so that the callback error
code is not converted into GIT_EUSER and instead we propagate the
return value through to the caller.  Instead of using the
giterr_capture and giterr_restore functions, we now rely on all
functions to pass back the return value from a callback.

To avoid having a return value with no error message, the user
can call the public giterr_set_str or some such function to set
an error message.  There is a new helper 'giterr_set_callback'
that functions can invoke after making a callback which ensures
that some error message was set in case the callback did not set
one.

In places where the sign of the callback return value is
meaningful (e.g. positive to skip, negative to abort), only the
negative values are returned back to the caller, obviously, since
the other values allow for continuing the loop.

The hardest parts of this were in the checkout code where positive
return values were overloaded as meaningful values for checkout.
I fixed this by adding an output parameter to many of the internal
checkout functions and removing the overload.  This added some
code, but it is probably a better implementation.

There is some funkiness in the network code where user provided
callbacks could be returning a positive or a negative value and
we want to rely on that to cancel the loop.  There are still a
couple places where an user error might get turned into GIT_EUSER
there, I think, though none exercised by the tests.

This continues auditing all the places where GIT_EUSER is being
returned and making sure to clear any existing error using the
new giterr_user_cancel helper.  As a result, places that relied
on intercepting GIT_EUSER but having the old error preserved also
needed to be cleaned up to correctly stash and then retrieve the
actual error.

Additionally, as I encountered places where error codes were not
being propagated correctly, I tried to fix them up.  A number of
those fixes are included in the this commit as well.

This adds `git_config__lookup_entry` which will look up a key in
a config and return either the entry or NULL if the key was not
present.  Optionally, it can either suppress all errors or can
return them (although not finding the key is not an error for this
function).  Unlike other accessors, this does not normalize the
config key string, so it must only be used when the key is known
to be in normalized form (i.e. all lower-case before the first dot
and after the last dot, with no invalid characters).

This also adds three high-level helper functions to look up config
values with no errors and a fallback value.  The three functions
are for string, bool, and int values, and will resort to the
fallback value for any error that arises.  They are:

* `git_config__get_string_force`
* `git_config__get_bool_force`
* `git_config__get_int_force`

None of them normalize the config `key` either, so they can only
be used for internal cases where the key is known to be in normal
format.

1. internal iterators now return GIT_ITEROVER when you go past the
   last item in the iteration.
2. git_iterator_advance will "advance" to the first item in the
   iteration if it is called immediately after creating the
   iterator, which allows a simpler idiom for basic iteration.
3. if git_iterator_advance encounters an error reading data (e.g.
   a missing tree or an unreadable file), it returns the error
   but also attempts to advance past the invalid data to prevent
   an infinite loop.

Updated all tests and internal usage of iterators to account for
these new behaviors.