Add support for diffing index with no HEAD

When a repo is first created, there is no HEAD yet and attempting
to diff files in the index was showing nothing because a tree
iterator could not be constructed.  This adds an "empty" iterator
and falls back on that when the head cannot be looked up.

Clean commit and tag messages

Enhancing the blob diffing experience

Add git_remote_add() and change signature for _new()

Add a fetch refspec arguemnt and make the arguments (name, url,
refspec), as that order makes more sense.

Helper function to create a remote with the default settings

Change the order and set a NULL so we don't try to access freed memory
in case of an error.

Improve the interop with bindings

Clear the error in pkt when we notice that the remote is starting to
send the packfile.

Fix the format string for Windows networking errors.

repository: ensure git_repository_discover() returns ENOTFOUND when unable to find a repository given the constraints

repository: ensure git_repository_open() returns ENOTFOUND when being passed a path leading to no repository

compat: make p_realpath Windows implementation be a bit more POSIX compliant and fail if the provided path does not lead to an existing entry

object: make git_object_lookup() return GIT_ENOTFOUND when searching for an existing object by specifying an incorrect type

remote: make git_remote_load() return GIT_ENOTFOUND when the remote url cannot be retrieved from the config file

More mingw32 compilation fixes.

The fix to support attrs on bare repos went a little too far
in trying to avoid using the working directory and ended up
not processing the input path quite correctly.

compat: make p_open able to accept optional mode when passing the O_CREAT flag

This has the nice side effect of making test_attr_repo__staging_properly_normalizes_line_endings_according_to_gitattributes_directives() test pass again on Windows. This test started to fail after commit 674a1985 was applied.

 - edf3dce -> assets.github.com/images/icons/emoji/alien.png?v5
 - de863bf -> assets.github.com/images/icons/emoji/heart.png?v5

diff: When diffing two blobs, ensure the delta callback parameter is filled with relevant information

'git commit' and 'git tag -a' enforce some conventions, like cleaning up excess whitespace and making sure that the last line ends with a '\n'. This fix replicates this behavior.

Fix libgit2/libgit2sharp#117

It can cause segfaults if the call didn't set an error

Previously, it was defined in netops.c, but it's also needed in one of the
clar tests, so I figured we might as well just make it global for the
whole project.

Without it, the mingw32 linker won't resolve GetProcessId() (called from
the core/errors.c clar test) because of some conditionals in windows.h.

Fixes the mingw32 build issues.