Complete test coverage for makemigrations command

[Claude Paroz]

Currently the coverage report of the makemigrations management command is only 61%. Let's try our best to extend this to 100% before releasing it.
​http://ci.djangoproject.com/job/Django%20Coverage/HTML_Coverage_Report/_var_lib_jenkins_jobs_Django%20Coverage_workspace_django_core_management_commands_makemigrations.html

Not blocking for the RC.

[Andrew Godwin]

I question this being marked as a release blocker at all; while I love test coverage as much as the next person, it doesn't find all the bugs, and trying to attain 100% coverage on a command that's very difficult to test because of the fact that it does filesystem operations is just going to slow us down (in that we'll get much better bug finding and reporting from a release and real-world usage than from spending weeks striving for 100% coverage)

Unless there's a convincing argument otherwise, I'd like to demote this from release blocker back down to normal. If we get it before the release, great! If not, let's not delay it further.

[Aymeric Augustin]

100% coverage is rarely achievable (nor useful) for management commands. However, 41% is a bit low ;-)

[Huu Nguyen]

Opened up a pull request for this ​here. Improved code coverage from 61% to 98%.

[Tim Graham <timograham@…>]

In f851a954ac605c4a525761a18d2dcbfb6f8deeca:

Fixed #22602 -- Improved code coverage of makemigrations command tests.

[Tim Graham <timograham@…>]

In 8462883f9b16810fbc6175d78d7bb13a297debe9:

[1.7.x] Fixed #22602 -- Improved code coverage of makemigrations command tests.

Backport of f851a954ac from master