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 <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: alipman88

t/t3200-branch.sh

@@ -1287,6 +1287,7 @@ test_expect_success 'detect typo in branch name when using --edit-description' '
 '
 
 test_expect_success 'refuse --edit-description on unborn branch for now' '
+	test_when_finished "git checkout master" &&
 	write_script editor <<-\EOF &&
 		echo "New contents" >"$1"
 	EOF