This fixes several bugs, updates tests and docs, eliminates the
FILE* assumption in favor of printing callbacks for the diff patch
formatter helpers, and adds a "diff" example function that can
perform a diff from the command line.

This reworks the diff API to separate the steps of producing
a diff descriptions from formatting the diff.  This will allow
us to share diff output code with the various diff creation
scenarios and will allow us to implement rename detection as
an optional pass that can be run on a diff list.

This gets the basic plumbing in place for git_diff_blob.
There is a known issue where additional parameters like
the number of lines of context to display on the diff
are not working correctly (which leads one of the new
unit tests to fail).

This cleans up the various GCC compiler warnings with the
xdiff code that was copied in.

Since casting to void works to eliminate errors with unused
parameters on all platforms, avoid the various special cases.
Over time, it will make sense to eliminate the GIT_UNUSED
macro completely and just have GIT_UNUSED_ARG.

This is the initial import of the xdiff code (LGPL) from
core git as of rev f349b562086e2b7595d8a977d2734ab2ef9e71ef

This is a test that should replicate an issue that Peff
is setting with git attributes.  But the test doesn't fail.

Filters, yo

This adds some initial unit tests for file filtering and fixes
some simple bugs in filter application.

The point of having `GIT_ATTR_TRUE` and `GIT_ATTR_FALSE` macros is to be
able to change the way that true and false values are stored inside of
the returned gitattributes value pointer.

However, if these macros are implemented as a simple rename for the
`git_attr__true` pointer, they will always be used with the `==`
operator, and hence we cannot really change the implementation to any
other way that doesn't imply using special pointer values and comparing
them!

We need to do the same thing that core Git does, which is using a
function macro. With `GIT_ATTR_TRUE(attr)`, we can change
internally the way that these values are stored to anything we want.

This commit does that, and rewrites a large chunk of the attributes test
suite to remove duplicated code for expected attributes, and to
properly test the function macro behavior instead of comparing
pointers.

Comments soothe my soul.

Sane API for real-world usage.

Friendlier revision walking

It's not unusual to want the walker to act on HEAD, so add a
convencience function for the case that the user doesn't already have
a resolved HEAD reference.

git_revwalk_{push,hide}_glob() lets you push the OIDs of references
that match the specified glob. This is the basics for what git.git
does with the rev-list options --branches, --tags, --remotes and
--glob.

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.