examples: cast away constness for reallocating head arrays

When reallocating commit arrays in `opts_add_commit` and
`opts_add_refish`, respectively, we simply pass the const pointer to
`xrealloc`. As `xrealloc` expects a non-const pointer, though, this will
generate a warning with some compilers.

Cast away the constness to silence compilers.

examples/describe.c

@@ -54,7 +54,7 @@ static void opts_add_commit(describe_options *opts, const char *commit)
 	assert(opts != NULL);
 
 	sz = ++opts->commit_count * sizeof(opts->commits[0]);
-	opts->commits = (const char **)xrealloc(opts->commits, sz);
+	opts->commits = xrealloc((void *) opts->commits, sz);
 	opts->commits[opts->commit_count - 1] = commit;
 }
 

examples/merge.c

@@ -57,7 +57,7 @@ static void opts_add_refish(merge_options *opts, const char *refish)
 	assert(opts != NULL);
 
 	sz = ++opts->heads_count * sizeof(opts->heads[0]);
-	opts->heads = (const char **)xrealloc(opts->heads, sz);
+	opts->heads = xrealloc((void *) opts->heads, sz);
 	opts->heads[opts->heads_count - 1] = refish;
 }