Initial implementation of gitignore support

git_status_foreach() and git_status_file() will now be
gitignore aware.

After reviewing the gitignore support with Vicent, we came up
with a list of minor cleanups to prepare for merge, including:

* checking git_repository_config error returns
* renaming git_ignore_is_ignored and moving to status.h
* fixing next_line skipping to include \r skips
* commenting on where ignores are and are not included

We force stat to be a stat64 structure, so we can and should put all
64 bits of the size in st_size.

off_t is always 32 bits in Windows, which is beyond stupid, but we just
don't care anymore because we're using `git_off_t` which is assured to
be 64 bits on all platforms, regardless of compilation mode. Just
ensure that no casts to `off_t` are performed.

Also, the check for `off_t` overflows has been dropped, once again,
because the size of our offsets is always 64 bits on all platforms.

Fixes #534

In the main loop we peek to see what kind of line the next one is. If
there are multiple newlines before the end of the file, the eof marker
won't be set after we read the last line with data and we'll try to
peek again. This peek will return LF (as it pretends that we have a
newline at EOF so other function don't need any special handling).

Fix cfg_getchar so it doesn't try to read past the last character in
the file and config_parse so it considers LF as EOF on peek (as we're
ignoring spaces) and sets the reader's EOF flag to exit the parsing
loop.

This contains fixes for several issues discovered by MSVC and
by valgrind, including some bad data access, some memory
leakage (in where certain files were not being successfully
added to the cache), and some code simplification.

This gets rid of the crazy macro version of git_path_walk_up
and makes it into a normal function that takes a callback
parameter.  This turned out not to be too messy.

The last checkin accidentally broke dir_for_path by propogating
the dirname return code even when there was no error.

This fixes issue 532 that attributes (and gitignores) could not
be checked for files that don't exist.  It should be possible to
query such things regardless of the existence of the file.

Adds support for .gitignore files to git_status_foreach() and
git_status_file().  This includes refactoring the gitattributes
code to share logic where possible.  The GIT_STATUS_IGNORED flag
will now be passed in for files that are ignored (provided they
are not already in the index or the head of repo).

It turns out that passing NULL for the second parameter of realpath(3)
is not as portable as one might like.  Notably, Mac OS 10.5 and earlier
does not support it.  So this moves us back to a large buffer to get
the realpath info.

Valgrind fixes in smaller pieces

Trees, indices and repos need to be freed

Missed freeing the entry.

This is the first time this error is throw, so use git__throw instead
of git__rethrow.

Calling git_repository_open immediately after git_repository_init
results in memory leaks.

We get the oid of a reference, free the reference, then
convert the oid to a string. We need to convert the oid
before freeing the memory.

Conflicts:
	tests-clay/clay.h
	tests-clay/clay_main.c

Fix some "attributes" related issues

The value returned by MultiByteToWideChar includes the NULL termination character.

Add missing semicolon

Conflicts:
	tests-clay/clay.h
	tests-clay/clay_main.c

Conflicts:
	tests-clay/clay_main.c

`delete` is a reserved keyword in C++.

This updates to implementation of gitattribute macros to be much more
similar to core git (albeit not 100%) and to handle expansion of
macros within macros, etc.  It also cleans up the refcounting usage
with macros to be much cleaner.

Also, this adds a new vector function `git_vector_insert_sorted()`
which allows you to maintain a sorted list as you go.  In order to
write that function, this changes the function `git__bsearch()` to
take a somewhat different set of parameters, although the core
functionality is still the same.

Currently, diff_index passes the full relative path from the
repository root to the callback. In case of an addition, it passes
the tree entry instead of the index entry.

This change fixes the path used for addition, and it passes only
the basename of the path. This mimics the current behavior of
git_tree_diff.

For the diff-index tests, the diff_more test will run multiple
times. Reset the expect_idx counter after each test in order to
allow this.

The git.git implementation allows this, and there is no reason not
to.

The clay test suite files clay.h and clay_main.c are generated by
the clay python script. Teach CMake about this dependency and
remove the generated files from the repository.

Add support for git attribute macro definitions.  Also, add
support for cache flush API to clear the attribute file content
cache when needed.

Additionally, improved the handling of global and system files,
making common utility functions in fileops and converting config
and attr to both use the common functions.

Adds a bunch more tests and fixed some memory leaks.  Note that
adding macros required me to use refcounted attribute assignment
definitions, which complicated, but probably improved memory usage.