refs: forbid clang to complain about unreachable code

When `NO_SYMLINK_HEAD` is defined, `create_ref_symlink()` is hard-coded
as `(-1)`, and as a consequence the condition `!create_ref_symlink()`
always evaluates to false, rendering any code guarded by that condition
unreachable.

Therefore, clang is _technically_ correct when it complains about
unreachable code. It does completely miss the fact that this is okay
because on _other_ platforms, where `NO_SYMLINK_HEAD` is not defined,
the code isn't unreachable at all.

Let's use the same trick as in 82e79c6 (git-compat-util: add
NOT_CONSTANT macro and use it in atfork_prepare(), 2025-03-17) to
appease clang while at the same time keeping the `-Wunreachable` flag
to potentially find _actually_ unreachable code.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

refs/files-backend.c

@@ -3183,7 +3183,13 @@ static int files_transaction_finish(struct ref_store *ref_store,
 		 * next update. If not, we try and create a regular symref.
 		 */
 		if (update->new_target && refs->prefer_symlink_refs)
-			if (!create_ref_symlink(lock, update->new_target))
+			/*
+			 * By using the `NOT_CONSTANT()` trick, we can avoid
+			 * errors by `clang`'s `-Wunreachable` logic that would
+			 * report that the `continue` statement is not reachable
+			 * when `NO_SYMLINK_HEAD` is `#define`d.
+			 */
+			if (NOT_CONSTANT(!create_ref_symlink(lock, update->new_target)))
 				continue;
 
 		if (update->flags & REF_NEEDS_COMMIT) {