Signed-off-by: Sven Strickroth <email@cs-ware.de>

(fixes issue #6089)

Signed-off-by: Sven Strickroth <email@cs-ware.de>

Fix repo init when template dir is non-existent

util: minor cleanup and refactoring to the date class

Instead of `git__date`, just use `git_date`.

cmake: re-enable WinHTTP

Ensure that we `enable_testing()` at the top-level CMakeLists.txt or
else we'll need to navigate within the build directory to the correct
place in the hierarchy to run `ctest`.  Now we can `ctest` at the
top-level again.

tag: set validity to 0 by default

`git_tag_name_is_valid` needs to set validity to 0 when it
short-circuits.

cmake: further refactorings

The select hashes module selects the hash; the CMakeLists.txt selects
the files to implement it.

Update the global variables `LIBGIT2_OBJECTS` to
`LIBGIT2_DEPENDENCY_OBJECTS` for clarity and consistency.

Apply the standard project cmake formatting to the modules.

When building under gcc 11, there is a warning about an incompatible pointer
type, since
[`__atomic_exchange`](https://gcc.gnu.org/onlinedocs/gcc/_005f_005fatomic-Builtins.html)
does not take `volatile` pointers:

```
In file included from ../src/common.h:81,
                 from ../src/transports/winhttp.c:8:
../src/thread-utils.h: In function ‘git___swap’:
../src/thread-utils.h:168:9: warning: argument 3 of ‘__atomic_exchange’ discards ‘volatile’ qualifier [-Wincompatible-pointer-types]
  168 |         __atomic_exchange(ptr, &newval, &foundval, __ATOMIC_SEQ_CST);
      |         ^~~~~~~~~~~~~~~~~
```

This change drops the `volatile` qualifier so that the pointer type matches
what `__atomic_exchange` expects.

When building under gcc 11, there is a warning about a misaligned guard clause
because there were mixed spaces and tabs:

```
[128/634] Building C object src/CMakeFiles/git2internal.dir/threadstate.c.o
../src/threadstate.c: In function ‘threadstate_dispose’:
../src/threadstate.c:39:5: warning: this ‘if’ clause does not guard... [-Wmisleading-indentation]
   39 |     if (threadstate->error_t.message != git_str__initstr)
      |     ^~
../src/threadstate.c:41:9: note: ...this statement, but the latter is misleadingly indented as if it were guarded by the ‘if’
   41 |         threadstate->error_t.message = NULL;
      |         ^~~~~~~~~~~
../src/threadstate.c: At top level:
```

This change indents the code with tabs for consistency with the rest of the
code, which makes the warning go away.

This mimics the behaviour of git which just prints a
warning and continues with the repo initialisation.

Add missing-declarations warning globally

We should enforce declarations throughout the code-base, including
examples, fuzzers and tests, not just in the `src` tree.

Older versions of gcc do not believe that we've adequately declared our
test functions.  Include `clar_suite.h` conditionally for those old
versions.  Do not do this on newer compilers to avoid unnecessary
recompilation of the entire suite when we add or remove a test function.