There are many paths through revparse that may return an error
code without reporting an error, I believe.  This fixes one of
them.  Because of the backtracking in revparse, it is pretty
complicated to fix the others.

A number of places were looking up option config values and then
not clearing the error codes if the values were not found.  This
moves the repeated pattern into a shared routine and adds the
extra call to giterr_clear() when needed.

There are some cases, particularly where no loaded ODB backends
support a particular operation, where we would return an error
code without having set an error.  This catches those cases and
reports that no ODB backends support the operation in question.

There were a couple of places where an error was being returned
from branch related code but no error message was being set.

Namespace support

Update diff handling of "untracked" directories

This includes more tests for various scenarios when diff includes
an untracked directory in the workdir with contents either ignored
or not.

When diff encounters an untracked directory, there was a shortcut
that it took which is not compatible with core git.  This makes
the default behavior no longer take that shortcut and instead look
inside the untracked directory to see if there are any untracked
files within it.  If there are not, then the directory is treated
as an ignore directory instead of an untracked directory.  This
has implications for the git_status APIs.

In preparation for more changes to the internal diff logic, it
seemed wise to split the very large git_diff__from_iterators into
separate functions that handle the four main cases (unmatched old
item, unmatched new item, unmatched new directory, and matched
old and new items).  Hopefully this will keep the logic easier to
follow even as more cases have to be added to this code.

Remove most inlines from the public API

Add refspec list accessors

Catch issue in config set with no config file

This prevents a segfault when setting a value in the config of a
repository that doesn't have a config file.

Bring back a way of acessing the git_refspec* from a remote.

Closes #1514

Removed useless prototype and renamed object typecast functions
declaration macro.

This removes the GIT_INLINE versions of the simple git_object
accessors and standardizes them with a helper macro in src/object.h
to build the function bodies.