update: always checkout master.

This should help to prevent situations where a user accidentally ends up
"stranded" in a branch indefinitely.

Additionally, the stash is never popped automatically at the end of a
successful update, but the stash-pop message is printed if something is
stashed.

When an interrupt occurs, the original behavior is still in place
(switch back to the old branch and revision, and pop the stash), though
this could be changed as well.

This commit modifies the behavior of both `brew update` and
"update-bash.sh" in the manner described above.

The idea for this approach is from Mike McQuaid.

Issue Homebrew/homebrew#48812

Closes Homebrew/homebrew#48993.

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