Test for gitmodules only submodule def

This should confirm that issue #835 is fixed where a submodule that
is only declared in the .gitmodules file was not accessible via the
submodule APIs.

This adds better header comments and also fixes a bug in one of
simple APIs that tells the number of lines in the current hunk.

This refactors the diff output code so that an iterator object
can be used to traverse and generate the diffs, instead of just
the `foreach()` style with callbacks.  The code has been rearranged
so that the two styles can still share most functions.

This also replaces `GIT_REVWALKOVER` with `GIT_ITEROVER` and uses
that as a common error code for marking the end of iteration when
using a iterator style of object.

Windows: Perform UTF-8 path conversion on the Stack

We don't care about the supposed zlib errors, and the leak from
giterr_set isn't interesting, as it gets freed each time an error is
set.

Give valgrind a suppressions file so it doesn't tell us about them.

Fix MSVC compilation warnings

repository: add failing repo initialization test

signature: make the OS give us the offset for git_signature_now

There is a better and less fragile way to calculate time offsets. Let
the OS take care of dealing with DST and simply take the the offset
between the local time and UTC that it gives us.

Passing SSL_VERIFY_PEER makes OpenSSL shut down the connection if the
certificate is invalid, without giving us a chance to ignore that
error. Pass SSL_VERIFY_NONE and call SSL_get_verify_result if the user
wanted us to check.

When no CNs match, we used to jump to on_error which gave a bogus
error as that's for OpenSSL errors. Jump to cert_fail so we tell the
user that the error came from checking the certificate.

Conflicts:
	include/git2/refs.h

Make git_object_peel a bit smarter

refs: expose git_reference_normalize_name()

revwalk: refuse push of non-commit objects

This expands the types of peeling that `git_object_peel` knows
how to do to include TAG -> BLOB peeling, and makes the errors
slightly more consistent depending on the situation.  It also
adds a new special behavior where peeling to ANY will peel until
the object type changes (e.g. chases TAGs to a non-TAG).

Using this expanded peeling, this replaces peeling code that was
embedded in `git_tag_peel` and `git_reset`.

branch: reduce code duplication

Check the type of the pushed object immediately instead of starting the
walk and failing in between.

It's not really needed with the current code as we have EOS and the
sideband's flush to tell us we're done.

Keep the distinction between processed and received objects.

This poor transport was forgotten in the recent sideband support.

Make sure that idx->pack isn't NULL before trying to free resources
under it.

Revparse: GIT_EAMBIGUOUS

Revparse now returns EAMBIGUOUS if the the spec
doesn't match any refs/tags, and is <4 characters.

Add sideband support

Submodule extensions