update: always report changes to branch 'master'

If the user's working copy is *not* on the 'master' branch, the revision
recorded after the update to the 'master' branch is not the one of the
updated 'master' branch but that of whatever the user's current branch
is. This tends to be not very helpful.

This change records the revision *after* the update to the 'master'
branch, but *before* switching back to the user's current branch prior
to the update, effectively causing the report to show changes to the
'master' branch.

Closes Homebrew/homebrew#42074.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>