Sane API for real-world usage.

Initial implementation. The relevant code is in `blob.c`: the blob write
function has been split into smaller functions.

	- Directly write a file to the ODB in streaming mode

	- Directly write a symlink to the ODB in direct mode

	- Apply a filter, and write a file to the ODB in direct mode

When trying to write a file, we first call `git_filter__load_for_file`,
which populates a filters array with the required filters based on the
filename.

If no filters are resolved to the filename, we can write to the ODB in
streaming mode straight from disk. Otherwise, we load the whole file in
memory and use double-buffering to apply the filter chain. We finish
by writing the file as a whole to the ODB.

This makes so much sense that I can't believe it hasn't been done
before. Kill the old `git_fbuffer` and read files straight into
`git_buf` objects.

Also: In order to fully support 4GB files in 32-bit systems, the
`git_buf` implementation has been changed from using `ssize_t` for
storage and storing negative values on allocation failure, to using
`size_t` and changing the buffer pointer to a magical pointer on
allocation failure.

Hopefully this won't break anything.

Add git_remote_list()

Loops through the configuration and generates a list of configured
remotes.

We used to erroneously consider "^$" as a special case for appending a
value to a multivar. This was a misunderstanding and we should always
append a value if there are no existing values that match.

While we're in the area, replace all the variables in-memory in one
swoop and then replace them on disk so as to avoid matching a value
we've just introduced.

We used to consider it an error if a remote didn't have at least a
fetch refspec. This was too much checking, as a remote doesn't in fact
need to have anything other than an URL configured to be considered
a remote.

Making a commit that results in a blob that already exists in the ODB (i.e.
committing something, then making a revert commit) will result in us trying
to p_rename -> MoveFileExW a temp file into the existing ODB entry. Despite
the MOVEFILE_REPLACE_EXISTING flag is passed in, Win32 does not care and
fails it with STATUS_ACCESS_DENIED.

To fix this, we p_unlink the ODB entry before attempting to rename it. This
call will typically fail, but we don't care, we'll let the p_rename fail if
the file actually does exist and we couldn't delete it for some reason (ACLs,
etc).

tests-clar/notes: init oid before using

Reported-by: Carlos Martín Nieto <carlos@cmartin.tk>
Signed-off-by: schu <schu-github@schulog.org>

If write_tree() returs an error, we used to set the error message and
continued looping. Exit the loop so we return the error.

This test is fragile if you leave extra files in the test
data directory, such as a foo.c~ file from editing with
Emacs.  Who would do such a thing?

This fixes the missing readdir_r from win32 and fixes other
platforms to always use the reentrant readdir_r form for reading
directory contents.

Fix -Wuninitialized warning

Signed-off-by: schu <schu-github@schulog.org>

Uniform iterators for trees, index, and workdir

This update addresses all of the feedback in pull request #570.

The biggest change was to create actual linked list stacks for
storing the tree and workdir iterator state.  This cleaned up
the code a ton.  Additionally, all of the static functions had
their 'git_' prefix removed, and a lot of other unnecessary
changes were removed from the original patch.

This makes two changes to iterator behavior: first, advance
can optionally do the work of returning the new current value.
This is such a common pattern that it really cleans up usage.

Second, for workdir iterators, this removes automatically
iterating into directories.  That seemed like a good idea,
but when an entirely new directory hierarchy is introduced
into the workdir, there is no reason to iterate into it if
there are no corresponding entries in the tree/index that it
is being compared to.

This second change actually wasn't a lot of code because not
descending into directories was already the behavior for
ignored directories.  This just extends that to all directories.

Add config multivar support

Export parse_tag_buffer as git_tag__parse_buffer.

This create a new git_iterator type of object that provides a
uniform interface for iterating over the index, an arbitrary
tree, or the working directory of a repository.

As part of this, git ignore support was extended to support
push and pop of directory-based ignore files as the working
directory is being traversed (so the array of ignores does
not have to be recreated at each directory during traveral).

There are a number of other small utility functions in buffer,
path, vector, and fileops that are included in this patch
that made the iterator implementation cleaner.

Fix copy-paste error

Remotes improvements

Allow setting the fetch and push refspecs, which is useful for
creating new refspecs.

Windows doesn't support POSIX regex, so we need to include it
ourselves. The sources come from git, which in turn took them from
gawk.