Fixes textmate#3

Previously we would only update marks when saving a document.

Currently TextMate will also call our command when creating a new untitled document (in a project which use SCM) so we have to exit early if there is no file name.

Requires TextMate 2.0-beta.9.3

This is required starting with TextMate 2.0-beta.9.3.

Previously it was difficult to tell the added and the changed image apart, especially when scrolling through source code.

On recent OS versions the /usr/bin/git executable will prompt the user to install git (if Xcode is not already installed) which can cause some user confusion, so we now avoid using /usr/bin/git unless Xcode is installed.

This would open the external program on save, setting the `no-ext-diff` disables this feature.

Requires TextMate 2.0-beta.12.12

When the document uses an encoding different from UTF-8, the output of `diff_cmd` may contain characters that are not valid UTF-8. This leads to following error:

    Update Gutter:38:in `block in <main>': invalid byte sequence in UTF-8 (ArgumentError)
    	from Update Gutter:35:in `open'
    	from Update Gutter:35:in `<main>'

This is especially annoying because this error pops up every time the document is saved (because the “Update Gutter” command runs on every save).

Setting `ascii-8bit` encoding directly on the io object works around the issue as we only care about the beginning part of the line not the actual contents. Fixes textmate#7.

For documents which have status other than none (or unknown) the command will already be called due to ‘callback.document.did-change-scm-status’.

Requires TextMate 2.0-rc.20.