The export command didn't output the diffs in color, even when color support
was enabled. This patch fixes that by making the export command use the default
ui.write method, instead of directly manipulating the ui.fout file object.
Also added a test case to verify color output to test-export.t.
Many tests didn't change back from subdirectories at the end of the tests ...
and they don't have to. The missing 'cd ..' could always be added when another
test case is added to the test file.
This change do that tests (99.5%) consistently end up in $TESTDIR where they
started, thus making it simpler to extend them or move them around.