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

reference_rename() cleanup

Fix typos in the README file

Never use the Qt trade mark with both letters capitalized, in the form
"QT".
(http://qt-project.org/trademarkpolicy.html)

Networking improvements