sequencer: fix error message on failure to copy SQUASH_MSG

The message talked about renaming, while the actual action is copying.
This was introduced by 6e98de7 ("sequencer (rebase -i): add support
for the 'fixup' and 'squash' commands", 2017-01-02).

Signed-off-by: Oswald Buddenhagen <[email protected]>
Acked-by: Phillip Wood <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: ossilator

sequencer.c

@@ -2311,7 +2311,7 @@ static int do_pick_commit(struct repository *r,
 			const char *dest = git_path_squash_msg(r);
 			unlink(dest);
 			if (copy_file(dest, rebase_path_squash_msg(), 0666)) {
-				res = error(_("could not rename '%s' to '%s'"),
+				res = error(_("could not copy '%s' to '%s'"),
 					    rebase_path_squash_msg(), dest);
 				goto leave;
 			}