Bail out of remote loading if the URL would be NULL

Apparently git_remote_list() includes even remotes for which git_remote_load() would fail. Sorry @nulltoken, false alarm.

This reverts commit f358ec14.

CC @nulltoken

Fix state when HEAD is not detached

Let the user grab the packfile as it's being written

This fixes a crash from attempting to invoke git__strdup() against NULL.

Fix bytes_received in fetch tests - we weren't calling the callback

Initial implementation of diff rename detection

Free conflict index entries on removal

Prefer GetLastError() for GITERR_OS on Win32

Improve docs, examples, warnings

Some more changes to the Index API

This improves docs in some of the public header files, cleans
up and improves some of the example code, and fixes a couple
of pedantic warnings in places.

Why Apple, why?

Update of text stats calculation

Do not interpret 0x85 as Next Line (NEL)
char when gathering statistics for a text file.

Reorganize transport architecture

Let the user get each object as a buffer+size pair so they can handle
the packfile content as they need to.