Allow users to specify additional repository extensions that they want
to support.  For example, callers can specify that they support
`preciousObjects` and then may open repositories that support
`extensions.preciousObjects`.

Similarly, callers may opt out of supporting extensions that the library
itself supports.

Introduce a utility function that compares a NUL terminated string to a
possibly not-NUL terminated string with length.  This is similar to
`strncmp` but with an added check to ensure that the lengths match (not
just the `size` portion of the two strings).

Also, add `git_commit_graph_writer_options_init`!

GCC C11 warnings

Instead of buf->"typeofbuffer"ReparseBuffer the members will be
referenced with buf->ReparseBuffer."typeofbuffer"

https://docs.microsoft.com/en-us/windows-hardware/drivers/ddi/ntifs/ns-ntifs-_reparse_data_buffer?redirectedfrom=MSDN
calls the union DUMMYUNIONNAME but that looks a bit cluttered.

This adds a `-Wunused-result`-proof `GIT_UNUSED()`, just to demonstrate
that it works. With this, sortedcache.h is now completely
`GIT_WARN_UNUSED_RESULT`-annotated!

This change adds the GIT_WARN_UNUSED_RESULT annotation, which makes the
compiler warn when a return result is not used. This avoids bugs.

Passes w/ gcc 11 on Fedora x64.

Protip: So you don;t have to suffer,

```
perl -pe 's/(-?(?:0x)?[A-Fa-f0-9]+)([Uu])?[Ll][Ll]/\U$2INT64_C(\E$1)/mg'
```

The `git_buf_text` namespace is unnecessary and strange.  Remove it,
just keep the functions prefixed with `git_buf`.