Prevent segfaulting on Windows when building a temporary filename

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

Conflicts:
	tests-clay/clay_main.c

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.

Signed-off-by: Carlos Martín Nieto <carlos@cmartin.tk>
Signed-off-by: Vicent Marti <tanoku@gmail.com>

# Please enter the commit message for your changes. Lines starting
# with '#' will be ignored, and an empty message aborts the commit.
#
# Author:    Carlos Martín Nieto <carlos@cmartin.tk>
#
# On branch development
# Your branch is ahead of 'origin/development' by 11 commits.
#
# Changes to be committed:
#   (use "git reset HEAD^1 <file>..." to unstage)
#
#	modified:   include/git2/tree.h
#	modified:   src/tree.c
#	modified:   tests-clay/clay_main.c
#	modified:   tests-clay/object/tree/diff.c
#
# Untracked files:
#   (use "git add <file>..." to include in what will be committed)
#
#	0001-remote-Cleanup-the-remotes-code.patch
#	466.patch
#	466.patch.1
#	488.patch
#	Makefile
#	libgit2.0.15.0.dylib
#	libgit2.0.dylib
#	libgit2.dylib
#	libgit2_clay
#	libgit2_test
#	tests-clay/object/tree/

Signed-off-by: Carlos Martín Nieto <carlos@cmartin.tk>
Signed-off-by: Vicent Marti <tanoku@gmail.com>

For each difference in the trees, the callback gets called with the
relevant information so the user can fill in their own data
structures.

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

Fix compilation warnings

Optimized implementation of git_buf_join

This streamlines git_buf_join and removes the join-append behavior,
opting instead for a very compact join-replace of the git_buf contents.
The unit tests had to be updated to remove the join-append tests and
have a bunch more exhaustive tests added.

Make git_buf functions always maintain a valid cstr

Taking a page from core git's strbuf, this introduces git_buf_initbuf
which is an empty string that is used to initialize the git_buf ptr
value even for new buffers.  Now the git_buf ptr will always point to
a valid NUL-terminated string.

This change required jumping through a few hoops for git_buf_grow
and git_buf_free to distinguish between a actual allocated buffer
and the global initial value.  Also, this moves the allocation
related functions to be next to each other near the top of buffer.c.

At a tiny cost of 1 extra byte per allocation, this makes
git_buf_cstr into basically a noop, which simplifies error
checking when trying to convert things to use dynamic allocation.

This patch also adds a new function (git_buf_copy_cstr) for copying
the cstr data directly into an external buffer.

Minor fixups

Actually add the tests introduced with 8c74d22e.

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

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

Extend git_buf with new utility functions and unit tests.

* replace some ints with size_ts
* update NULL checks in various places

This commit addresses two of the comments:
* renamed existing n-input git_buf_join to git_buf_join_n
* added new git_buf_join that always takes two inputs
* moved some parameter error checking to asserts
* extended unit tests to cover new version of git_buf_join

- Hide the remaining transports code
- Drop `git_headarray`, switch to using a callback to list refs. Makes
the code cleaner.

Add new functions to git_buf for:
* initializing a buffer from a string
* joining one or more strings onto a buffer with separators
* swapping two buffers in place
* extracting data from a git_buf (leaving it empty)

Also, make git_buf_free leave a git_buf back in its initted state,
and slightly tweak buffer allocation sizes and thresholds.

Finally, port unit tests to clay and extend with lots of new tests
for the various git_buf functions.

Thanks @carlosmn!

There's no difference between `_free` and `_close` semantics: keep
everything with the same name to avoid confusions.

The ownership semantics have been changed all over the library to be
consistent. There are no more "borrowed" or duplicated references.

Main changes:

	- `git_repository_open2` and `3` have been dropped.

	- Added setters and getters to hotswap all the repository owned
	objects:

		`git_repository_index`
		`git_repository_set_index`
		`git_repository_odb`
		`git_repository_set_odb`
		`git_repository_config`
		`git_repository_set_config`
		`git_repository_workdir`
		`git_repository_set_workdir`

	Now working directories/index files/ODBs and so on can be
	hot-swapped after creating a repository and between operations.

	- All these objects now have proper ownership semantics with
	refcounting: they all require freeing after they are no longer
	needed (the repository always keeps its internal reference).

	- Repository open and initialization has been updated to keep in
	mind the configuration files. Bare repositories are now always
	detected, and a default config file is created on init.

	- All the tests affected by these changes have been dropped from the
	old test suite and ported to the new one.

Don't fail when opening a new config file