Only write files when content changes

[davidtaylorhq]

Previously the CLI would call File.write for every file, even if the contents was unchanged. This unnecessary filesystem churn can have a knock-on effect on other tools which may be watching directories for changes (e.g. IDEs). This commit updates the stree write command so that it only performs a write when the file contents has changed.

There is also a very slight performance improvement in the 'happy path' when files are already formatted. Measuring runtime on Discourse's core codebase (4.3k ruby files), when all files are already formatted:

	run 1	run 2	run 3	mean
Before	26.4	27.1	26.1	26.5s
After	24.3	23.8	24.4	24.2s

Presumably this difference would be larger in environments with slower filesystems.

[kddnewton]

Ahh thank you this is a great change!