diff::parse: don't include `diff.h`

We don't call any internal functions in the test; we don't need to
include `../src/diff.h`.

release script: fix typo

tests: offer exact name matching with a `$` suffix

httpclient: support googlesource

git_packbuilder_write: Allow setting path to NULL to use the default path

mempack: Use threads when building the pack

When using `-s` to specify a particular test, it will do a prefix match.
Thus, `-sapply::both::rename_a_to_b_to_c` will match both a test named
`test_apply_both__rename_a_to_b_to_c` and a test that begins with that
name, like `test_apply_both__rename_a_to_b_to_c_exact`.

Permit a trailing `$` to `-s` syntax.  This allows a user to specify
`-sapply::both::rename_a_to_b_to_c$` to match _only_ the
`test_apply_both__rename_a_to_b_to_c` function.

We already filter to ensure that the given prefix matches the current
test name.  Also ensure that the length of the test name matches the
length of the filter, sans trailing `$`.

clar: use internal functions instead of /bin/cp and /bin/rm

Similar to how clar has used `/bin/cp` to copy files, it's used
`/bin/rm` to remove them.  This has similar deficiencies; meaning that
leaks is noisy and it's slow.  Move it to an internal function.

clar has historically shelled out to `/bin/cp` to copy test fixtures
into a sandbox.  This has two deficiencies:

1. It's slower than simply opening the source and destination and
   copying them in a read/write loop.  On my Mac, the `/bin/cp` based
   approach takes ~2:40 for a full test pass.  Using a read/write loop
   to copy the files ourselves takes ~1:50.

2. It's noisy.  Since the leak detector follows fork/exec, we'll end up
   running the leak detector on `/bin/cp`.  This would be fine, except
   that the leak detector spams the console on startup and shutdown, so
   it adds a _lot_ of additional information to the test runs that is
   useless.  By not forking and using this internal system, we see much
   less output.

strarray refactoring

The httpclient implementation keeps a `read_buf` that holds the data
in the body of the response after the headers have been written.  We
store that data for subsequent calls to `git_http_client_read_body`.  If
we want to stop reading body data and send another request, we need to
clear that cached data.

Clear the cached body data on new requests, just like we read any
outstanding data from the socket.

When `git_http_client_read_body` is invoked, it provides the size of the
buffer that can be read into.  This will be set as the parser context's
`output_size` member.  Use this as an upper limit on our reads, and
ensure that we do not read more than the client requests.

We should not be in the business of copying strings around for users.
We either return a strarray that can be freed, or we take one (and do
not mutate it).

We _dispose_ the contents of objects; we _free_ objects (and their
contents).  Update `git_strarray_free` to be `git_strarray_dispose`.
`git_strarray_free` remains as a deprecated proxy function.

When users call `git_http_client_read_body`, it should return 0 at the
end of a message.  When the `on_message_complete` callback is called,
this will set `client->state` to `DONE`.  In our read loop, we look for
this condition and exit.

Without this, when there is no data left except the end of message chunk
(`0\r\n`) in the http stream, we would block by reading the three bytes
off the stream but not making progress in any `on_body` callbacks.
Listening to the `on_message_complete` callback allows us to stop trying
to read from the socket when we've read the end of message chunk.

Google Git (googlesource.com) behaves differently than git proper.
Test that we can communicate with it.

CMake cleanups

git_pool_init: allow the function to fail

Propagate failures caused by pool initialization errors.

We currently disable deprecation synchronization warnings in case we're
building with Clang. We check for Clang by doing a string comparison on
the compiler identification, but this seems to have been broken by an
update in macOS' image as the compiler ID has changed to "AppleClang".
Let's just unconditionally disable this warning on Unix platforms. We
never add the deprecated attribute anyway, so the warning doesn't help
us at all.

The `CMAKE_MINIUM_REQUIRE()` function not only sets up the minimum
required CMake version of a project, but it will also at the same time
set the CMake policy version. In effect this means that all policies
that have been introduced before the minimum CMake version will be
enabled automatically.

When updating our minimum required version ebabb88f (cmake: update
minimum CMake version to v3.5.1, 2019-10-10), we didn't remove any of
the policies we've been manually enabling. The newest CMake policy we've
been enabling is CMP0054, which was introduced back in CMake v3.1. As a
result, we can now just remove all manual calls to `CMAKE_POLICY()`.

We currently have an option that adds options for profiling to both our
CFLAGS and LDFLAGS. Having such flags behind various build options is
not really sensible at all, since users should instead set up those
flags via environment variables supported by CMake itself.

Let's remove this option.

We currently have support for generating tags via ctags as part of our
build system. We aren't really in the place of supporting any tooling
that exists apart from the actual build environment, as doing so adds
additional complexity and maintenance burden to our build instructions.
This is in fact nicely demonstrated by this particular option, as it
hasn't been working anymore since commit e5c9723d (cmake: move library
build instructions into subdirectory, 2017-06-30).

As a result, this commit removes support for building CTags

Our custom CMake module currently live in "cmake/Modules". As the
"cmake/" directory doesn't contain anything except the "Modules"
directory, it doesn't really make sense to have the additional
intermediate directory. So let's instead move the modules one level up
into the "cmake/" top level directory.

diff::workdir: actually test the buffers

Handle unreadable configuration files

Modified `config_file_open()` so it returns 0 if the config file is
not readable, which happens on global config files under macOS
sandboxing (note that for some reason `access(F_OK)` DOES work with
sandboxing, but it is lying). Without this read check sandboxed
applications on macOS can not open any repository, because
`config_file_read()` will return GIT_ERROR when it cannot read the
global /Users/username/.gitconfig file, and the upper layers will
just completely abort on GIT_ERROR when attempting to load the
global config file, so no repositories can be opened.

Make git_index_write() generate valid v4 index

The `git_index_free()` merely decrement the reference counter from 2 to
1, and does not "free" the index.
Thus, the following `git_repository_index()` merely increase the counter
to 2, instead of read index from disk.
The written index is not read and parsed, which makes this test case
effectively becomes a no-op.

According to index-format.txt of git, the path of an entry is prefixed
with N, where N indicates the length of bytes to be stripped.

If given a NULL path, write to the object path of the repository.

Add tests for the new behavior.

Clean up and return via a single label, to avoid duplicate error
handling before each return, and to make it easier to extend the set of
cleanups needed.

The mempack ODB backend creates a packbuilder internally to write out a
pack; call git_packbuilder_set_threads on that packbuilder, to use
threads for packing if available.