Use list of arguments for execute_command, not a string.

[htgoebel]

This avoids wrong quoting in arguments if one e.g. passes a double-quote in a message.

• Convert all command-strings into lists.
• New function utils.excute_commands() (plural).
• New function utils.format_commands().
• Incompatible: git.cmd_checkout_from_tag() now returns a list of commands, not a single one.
• Adjusted tests.

Closes #233.

[htgoebel]

Tests pass, ready to merge.

Thanks to the extensive test-suite, it made working out bugs a breeze.

[htgoebel]

Huh! What a stupid bug! Looks like this is not covered by a test-case.

[reinout]

Yes. That one was missing from the test case. It was easy to add, though. Fixed in the mean time.