Attrcache cleanups

Provide error on gcc < 4.1 when THREADSAFE

Provide a descriptive error message when compiling THREADSAFE on gcc
versions < 4.1.  We require the atomic primitives (eg
`__sync_synchronize`) that were introduced in that version.

(Note, clang setes `__GNUC__` but appears to set its version > 4.1.)

Remove useless indirection from `git_attr_cache__init` to
`git_attr_cache__do_init`. The difference is that the
`git_attr_cache__init` macro first checks if the cache is already
initialized and, if so, not call `git_attr_cache__do_init`. But
actually, `git_attr_cache__do_init` already does the same thing and
returns immediately if the cache is already initialized.

Remove the indirection.

When doing an upsert of a file, we used to use `git__compare_and_swap`,
comparing the entry's file which is to be replaced with itself. This can
be more easily formulated by using `git__swap`, which unconditionally
replaces the value.

Improve encapsulation by not referencing the attrcache mutex directly
but instead using the `attr_cache_lock` and `attr_cache_unlock`
functions.

Improve clar messages

Provide more detailed messages when conditions pass or fail
unexpectedly.  In particular, this provides the error messages when a
test fails with a different error code than was expected.

`snprintf` requires a _format_ but does not require _arguments_ to the
format.  eg: `snprintf(buf, 42, "hi")` is perfectly legal.  Expand the
macro to match.

Without this, `p_sprintf(buf, 42, "hi")` errors with:

```
error: expected expression
                p_snprintf(msg, 42, "hi");
                ^
src/unix/posix.h:53:34: note: expanded from macro 'p_snprintf'
                                 ^
/usr/include/secure/_stdio.h:57:73: note: expanded from macro 'snprintf'
  __builtin___snprintf_chk (str, len, 0, __darwin_obsz(str),
__VA_ARGS__)
```

khash cleanups

refdb: catch additional per-worktree refs

Minor comment fix

The upstream git.git project currently identifies all references inside
of `refs/bisect/` as well as `HEAD` as per-worktree references. This is
already incorrect and is currently being fixed by an in-flight topic
[1]. The new behavior will be to match all pseudo-references outside of
the `refs/` hierarchy as well as `refs/bisect/`.

Our current behavior is to mark a selection of pseudo-references as
per-worktree, only. This matches more pseudo-references than current
git, but forgets about `refs/bisect/`. Adjust behavior to match the
in-flight topic, that is classify the following references as
per-worktree:

- everything outside of `refs/`
- everything inside of `refs/bisect/`

[1]: <20170213152011.12050-1-pclouds@gmail.com>

Signature cleanups