diff: ignore nsecs when diffing

Although our index contains the literal time present in the index,
we do not read nanoseconds from disk, and thus we should not use
them in any comparisons, lest we always think our working directory
is dirty.

Guard this behind a `GIT_USE_NSECS` for future improvement.

src/diff.c

@@ -493,8 +493,10 @@ static int diff_list_apply_options(
 
 	/* Don't set GIT_DIFFCAPS_USE_DEV - compile time option in core git */
 
-	/* Set GIT_DIFFCAPS_TRUST_NANOSECS on a platform basis */
+	/* Don't trust nanoseconds; we do not load nanos from disk */
+#ifdef GIT_USE_NSEC
 	diff->diffcaps = diff->diffcaps | GIT_DIFFCAPS_TRUST_NANOSECS;
+#endif
 
 	/* If not given explicit `opts`, check `diff.xyz` configs */
 	if (!opts) {