libcxx: "Fix" shallow runtimes build for clones

For a shallow clone we have no way to know whether the commit in
question is present or not, so we have to just make some sensible
assumptions, i.e. that it's current CHERI-LLVM. This gives false
negatives, but we can't really do much better without looking in the
actual source for signs that the requirements are met.

pycheribuild/projects/cross/libcxx.py

@@ -600,7 +600,8 @@ def configure(self, **kwargs) -> None:
                 self.add_cmake_options(LIBCXX_ENABLE_ASSERTIONS=True)
                 # Need to export the symbols from debug.cpp to allow linking code that defines _LIBCPP_DEBUG=1
                 self.add_cmake_options(LIBCXX_ENABLE_BACKWARDS_COMPATIBILITY_DEBUG_MODE_SYMBOLS=True)
-        if not GitRepository.contains_commit(self, "f4fdc4f4d9f6903808541645d383be2ee759f400", src_dir=self.source_dir):
+        if not GitRepository.contains_commit(self, "f4fdc4f4d9f6903808541645d383be2ee759f400", src_dir=self.source_dir,
+                                             invalid_commit_ref_result=True):
             self.fatal(f"LLVM revision in {self.source_dir} is too old for this target, please update.")
         super().configure(**kwargs)