t3200: clean side effect of git checkout --orphan

t3200: clean side effect of git checkout --orphan

The "refuse --edit-description on unborn branch for now" test in t3200
switches to an orphan branch, causing subsequent git commands
referencing HEAD to fail. Avoid this side-effect by switching back to
master after the test finishes.

This has gone undetected, as the next affected test expects failure -
but it currently fails for the wrong reason.

Verbose output of the next test referencing HEAD,
"--merged is incompatible with --no-merged":

  fatal: malformed object name HEAD

Which this commit corrects to:

  error: option `no-merged' is incompatible with --merged

Signed-off-by: Aaron Lipman <alipman88@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>