Build failure with -std=gnu23 (GCC 15 default)

[Sam James <sam@gentoo.org>]

Hi,

Upcoming GCC 15 defaults to -std=gnu23. git-2.47.0 fails to build as
follows:
```
builtin/index-pack.c:97:8: error: expected ‘{’ before ‘thread_local’
   97 | struct thread_local {
      |        ^~~~~~~~~~~~
builtin/index-pack.c:120:15: error: expected ‘{’ before ‘thread_local’
  120 | static struct thread_local nothread_data;
      |               ^~~~~~~~~~~~
builtin/index-pack.c:151:15: error: expected ‘{’ before ‘thread_local’
  151 | static struct thread_local *thread_data;
      |               ^~~~~~~~~~~~
```

There may be more issues, but at the very least thread_local became a
proper keyword in C23, so that will need renaming.

It should be possible to reproduce these with older GCC (and Clang) with
-std=gnu23 or -std=c23 set manually.

thanks,
sam