clar: add tap output option

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

Add an output abstraction layer, with a single output format, "clap",
the clar protocol, which is the current output format for clar.

We want to parse arguments before we start printing any output; the
arguments themselves may impact the way we display that output.

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