use same color for git-rebase-hash as for magit-log-sha1

These colors had differed for a long time.  Eventually I changed
`git-rebase-hash` (in git-rebase-mode.el) to match `magit-log-sha1`,
and shortly thereafter someone changed just the latter (see #158) in the theme
that I am using (this one), making these faces inconsistent again... grrrr