gc.sh: include size of packed-refs in reposizek
Previously this was excluded as being insignificant. However, the
repository would be unusable without a list of heads and their
references, so include the size of the packed-refs file along with
the size of the .pack file.
This total should be the same size as a `git bundle create foo --all`
file would be. In the case of repositories with 26000 (or more)
refs such as often occurs with Gerrit repositories, the packed-refs
file can actually be more than a megabyte in size, so not actually
an insignificant amount.