blame.c: fix garbled error message

The helper functions prepare_final() and prepare_initial() return a
pointer to a string that is a member of an object in the revs->pending
array. This array is later rebuilt when running prepare_revision_walk()
which potentially transforms the pointer target into a bogus string. Fix
this by maintaining a copy of the original string.

Signed-off-by: Lukas Fleischer <git@cryptocrack.de>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/blame.c

@@ -2390,7 +2390,7 @@ static struct commit *fake_working_tree_commit(struct diff_options *opt,
 	return commit;
 }
 
-static const char *prepare_final(struct scoreboard *sb)
+static char *prepare_final(struct scoreboard *sb)
 {
 	int i;
 	const char *final_commit_name = NULL;
@@ -2415,10 +2415,10 @@ static const char *prepare_final(struct scoreboard *sb)
 		sb->final = (struct commit *) obj;
 		final_commit_name = revs->pending.objects[i].name;
 	}
-	return final_commit_name;
+	return xstrdup_or_null(final_commit_name);
 }
 
-static const char *prepare_initial(struct scoreboard *sb)
+static char *prepare_initial(struct scoreboard *sb)
 {
 	int i;
 	const char *final_commit_name = NULL;
@@ -2445,7 +2445,7 @@ static const char *prepare_initial(struct scoreboard *sb)
 	}
 	if (!final_commit_name)
 		die("No commit to dig down to?");
-	return final_commit_name;
+	return xstrdup(final_commit_name);
 }
 
 static int blame_copy_callback(const struct option *option, const char *arg, int unset)
@@ -2489,7 +2489,7 @@ int cmd_blame(int argc, const char **argv, const char *prefix)
 	struct origin *o;
 	struct blame_entry *ent = NULL;
 	long dashdash_pos, lno;
-	const char *final_commit_name = NULL;
+	char *final_commit_name = NULL;
 	enum object_type type;
 
 	static struct string_list range_list;
@@ -2786,6 +2786,8 @@ parse_done:
 
 	assign_blame(&sb, opt);
 
+	free(final_commit_name);
+
 	if (incremental)
 		return 0;