Ensure HEAD is correct when skipping remote fetch

A recent optimization (2fb6466f) skips
performing a remote fetch if we already know we have the sha1 we want.
However, that optimization skipped initialization steps that ensure HEAD
points to the correct sha1.  This change makes sure not to skip those
steps.

Here is an example of how to test this change:

"""""""""
url=<manifest url>
branch1=<branch name>
branch2=<branch name>
project=<project with revision set to different sha1 in each branch>

repo init -u $url -b $branch1 --mirror
repo sync $project
first=$(cd $project.git; git rev-parse HEAD)

repo init -b $branch2
repo sync $project
second=$(cd platform/build.git; git rev-parse HEAD)

if [[ $first == $second ]]
then
    echo 'problem!'
else
    echo 'no problem!'
fi
"""""""""