Without this change, compiling with gcc and pedantic generates warning:
ISO C does not allow extra ‘;’ outside of a function.

The way we currently do it depends on the subtlety of strlen vs sizeof
and the fact that .pack is one longer than .idx. Let's use a git_buf so
we can express the manipulation we want much more clearly.

When creating an index, know that we do not have an index for
our own packfile, preventing some unnecessary file opens and
error reporting.

Keep the definitions in the headers, while putting the declarations in
the C files. Putting the function definitions in headers causes
them to be duplicated if you include two headers with them.

Fix race in git_packfile_unpack.

Increment refcount of newly added cache entries just like existing
entries looked up from the cache. Otherwise the new entry can be
evicted from the cache and destroyed while it's still in use.

Make our overflow checking look more like gcc and clang's, so that
we can substitute it out with the compiler instrinsics on platforms
that support it.  This means dropping the ability to pass `NULL` as
an out parameter.

As a result, the macros also get updated to reflect this as well.

Introduce some helper macros to test integer overflow from arithmetic
and set error message appropriately.