This takes all of the functions that look up simple data about
paths (such as `git_futils_isdir`) and moves them over to path.h
(becoming `git_path_isdir`).  This leaves fileops.h just with
functions that actually manipulate the filesystem or look at
the file contents in some way.

As part of this, the dir.h header which is really just for win32
support was moved into win32 (with some minor changes).

This converts virtually all of the places that allocate GIT_PATH_MAX
buffers on the stack for manipulating paths to use git_buf objects
instead.  The patch is pretty careful not to touch the public API
for libgit2, so there are a few places that still use GIT_PATH_MAX.

This extends and changes some details of the git_buf implementation
to add a couple of extra functions and to make error handling easier.

This includes serious alterations to all the path.c functions, and
several of the fileops.c ones, too.  Also, there are a number of new
functions that parallel existing ones except that use a git_buf
instead of a stack-based buffer (such as git_config_find_global_r
that exists alongsize git_config_find_global).

This also modifies the win32 version of p_realpath to allocate whatever
buffer size is needed to accommodate the realpath instead of hardcoding
a GIT_PATH_MAX limit, but that change needs to be tested still.

Update all stack allocations of git_filebuf to use GIT_FILEBUF_INIT
and make git_filebuf_open and git_filebuf_cleanup safe to be called
multiple times on the same buffer.

Signed-off-by: Vicent Marti <tanoku@gmail.com>

Move the callers of git_futils_mv_atomic to use p_rename.

Signed-off-by: Carlos Martín Nieto <carlos@cmartin.tk>

Create objects, indexes, and directories with the right file permissions

Ensure that all memory related functions (malloc, calloc, strdup, free,
etc) are using their respective `git__` wrappers.

The following files now have 0444 permissions:

- loose objects
- pack indexes
- pack files
- packs downloaded by fetch
- packs downloaded by the HTTP transport

And the following files now have 0666 permissions:

- config files
- repository indexes
- reflogs
- refs

This brings libgit2 more in line with Git.

Note that git_filebuf_commit() and git_filebuf_commit_at() have both
gained a new mode parameter.

The latter change fixes an important issue where filebufs created with
GIT_FILEBUF_TEMPORARY received 0600 permissions (due to mkstemp(3)
usage). Now we chmod() the file before renaming it into place.

Tests have been added to confirm that new commit, tag, and tree
objects are created with the right permissions. I don't have access to
Windows, so for now I've guarded the tests with "#ifndef GIT_WIN32".

To further match how Git behaves, this change makes most of the
directories libgit2 creates in a git repo have a file mode of
0777. Specifically:

- Intermediate directories created with git_futils_mkpath2file() have
  0777 permissions. This affects odb_loose, reflog, and refs.

- The top level folder for bare repos is created with 0777
  permissions.

- The top level folder for non-bare repos is created with 0755
  permissions.

- /objects/info/, /objects/pack/, /refs/heads/, and /refs/tags/ are
  created with 0777 permissions.

Additionally, the following changes have been made:

- fileops functions that create intermediate directories have grown a
  new dirmode parameter. The only exception to this is filebuf's
  lock_file(), which unconditionally creates intermediate directories
  with 0777 permissions when GIT_FILEBUF_FORCE is set.

- The test runner now sets the umask to 0 before running any
  tests. This ensurses all file mode checks are consistent across
  systems.

- t09-tree.c now does a directory permissions check. I've avoided
  adding this check to other tests that might reuse existing
  directories from the prefabricated test repos. Because they're
  checked into the repo, they have 0755 permissions.

- Other assorted directories created by tests have 0777 permissions.

filebuf.c: fix unused-but-set warning