Simplify git_futils_mkdir

There are two places where git_futils_mkdir should exit early or
at least do less.  The first is when using GIT_MKDIR_SKIP_LAST
and having that flag leave no directory left to create; it was
being handled previously, but the behavior was subtle.  Now I put
in a clear explicit check that exits early in that case.

The second is when there is no directory to create, but there is
a valid path that should be verified.  I shifted the logic a bit
so we'll be better about not entering the loop than that happens.

This routine was (is) pretty complicated, but given the recent
changes, it seemed like it could be simplified a bit.

Ensure git_futils_mkdir won't mkdir root

This makes sure that git_futils_mkdir always skips over the root
directory at a minimum, even on platforms where the root is not
simply '/'.  Also, this removes the GIT_WIN32 ifdef in favor of
making EACCES as a potentially recoverable error on all platforms.

Allow creation of directories under the volume root in Win32

We ran into an issue where cloning a repository to a folder
directly underneath the root of a volume (e.g. 'd:\libgit2')
would fail with an access denied error. This was traced down
to a call to make a directory that is the root (e.g. 'd:') could
return an error indicated access denied instead of an error
indicating the path already exists. This change now handles
the access denied error on Win32 and checks for the existence
of the folder.

Breaking RefDB changes

Don't bail on parsing commits with an invalid timezone

git doesn't do that, and it's not something that's usually
actionable to fix. if you have a git repository with one bad
timezone in the history, it's too late to change it most likely.

Make git_index_read_tree preserve stat cache where possible

Instead of just blowing away the stat cache data when loading a
new tree into the index, this checks if each loaded item has a
corresponding existing item with the same OID and if so, copies
the stat data from the old item to the new one so it will not be
blown away.

Make iterators use GIT_ITEROVER & smart advance

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.

improve test for index extension truncation

Added support for setting transport flags (No check SSL cert) to git_clo...

Introduce git_reference_shorthand

remote: make default tag retrieving behaviour consistent

Build with the system's http-parser installation if available

Default for newly created remotes will be auto.
Default when loading existing remotes with no tag retrieving behaviour set, was already auto.

thread: fix segfault on Windows 64 bits

`lpExitCode` is a pointer to a long. A long is 32 bits wide on Windows.

It means that on Windows 64bits, `GetExitCodeThread()` doesn't set/clear the high-order bytes of the 64 bits memory space pointed at by `value_ptr`.

allow (ignore) bare slash in gitignore

Fix memory leak in oid shortener tests

Fix two typos in the merge docs.