wt-status: do not depend on flaky reflog messages

b397ea4 (status: show more info than "currently not on any branch",
2013-03-13) made the output of 'git status' richer in the case of a
detached HEAD.  Before this patch, with a detached HEAD:

  $ git status
  # Not currently on any branch.

After the patch:

  $ git checkout v1.8.2
  # HEAD is now detached
  $ git status
  # HEAD detached at v1.8.2.

It works by digging the reflog for the most recent message of the form
"checkout: moving from xxxx to yyyy".  It then asserts that HEAD and
"yyyy" are the same, and displays this message.  When they aren't equal,
it displays:

  $ git status
  # HEAD detached from fe11db.

At this point, the utility of such a message is in question.  Moreover,
there are several tests in t/status-help that explicitly rely on rebase
writing "checkout: " messages to the reflog.  As evidenced by the
failing tests in t/checkout-last, these messages are completely
unintended and flaky.  Relying on them only makes it harder to improve
the reflog messages written by scripts.  As a reasonable compromise,
remove the logic to display the "HEAD detached from ..." message, and
fallback to "Not currently on any branch." in this case.  Update the
tests, giving scripts some breathing space.

This issue is not isolated to rebase at all.  Several other scripts like
bisect write (confusing) "checkout: " messages to the reflog.  Fixing
them is left as an exercise to other contributors.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
checkout: respect GIT_REFLOG_ACTION

GIT_REFLOG_ACTION is an environment variable specifying the reflog
message to write after an action is completed.  Several other commands
including merge, reset, and commit respect it.

Fix the failing tests in t/checkout-last by making checkout respect it
too.  You can now expect

  $ git checkout -

to work as expected after any rebase operation.

Also update the tests in t/status-help that rely on rebase writing
"checkout: " messages.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>