migrations.test_commands.MakeMigrationsTests fail on Windows when run on a different drive than C:

[Zbigniew Siciarz]

Some of the test cases in MakeMigrationsTests fail on Windows due to inability to find a relative path between files located on separate drives.
The tests create a temporary migrations directory using tempfile.mkdtemp, which on Windows creates a directory under C:\Temp (or C:\Users\<username>\AppData\Local\Temp).

If one clones Django source code to a drive other than C:, then the makemigrations command tests fail with an error like the following (Windows 7, Python 3.5.1):

ERROR: test_makemigrations_non_interactive_not_null_alteration (migrations.test_commands.MakeMigrationsTests)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "D:\dev\django\tests\migrations\test_commands.py", line 834, in test_makemigrations_non_interactive_not_null_alteration
    call_command("makemigrations", "migrations", interactive=False, stdout=out)
  File "d:\dev\django\django\core\management\__init__.py", line 130, in call_command
    return command.execute(*args, **defaults)
  File "d:\dev\django\django\core\management\base.py", line 330, in execute
    output = self.handle(*args, **options)
  File "d:\dev\django\django\core\management\commands\makemigrations.py", line 193, in handle
    self.write_migration_files(changes)
  File "d:\dev\django\django\core\management\commands\makemigrations.py", line 211, in write_migration_files
    migration_string = os.path.relpath(writer.path)
  File "D:\Miniconda3\lib\ntpath.py", line 574, in relpath
    path_drive, start_drive))
ValueError: path is on mount 'C:', start on mount 'D:'

Since the value returned from relpath is used only for display and not for any I/O, I'd suggest catching this ValueError and using an absolute path in that case.

[Zbigniew Siciarz]

The following pull request fixes this issue: ​https://github.com/django/django/pull/7472

[Simon Charette]

Left some comments for improvement.

[Simon Charette]

Patch and added test LGTM.

[Tim Graham <timograham@…>]

In f94ce0d2:

Fixed #27436 -- Fixed makemigrations crash when creating migrations on a separate drive than where Django is installed.