info: fix "current" output

The "Current revision" field shows the revision as listed in the
manifest.  I think most users expect this to show the revision
that the git tree is checked out to instead.  Switch the output
to show that revision instead, and add a "Current branch" if it
matches a local branch.

Change-Id: Ib546f5ebbc8a23875fbd14bf166fbe95b7dd244e
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/239240
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>