[WIP] Diff formatting

[olafurpg]

This PR introduces a new feature called "diff formatting". With scalafmt --diff, you only format the lines that are touched by a diff. Examples

# via stdin
$ git diff -U0 master | scalafmt --diff --stdin
... # writes to files
$ svn diff --diff-cmd=diff -x-U0 | scalafmt --diff --stdin # untested, but should work
# via configuration
$ cat .scalafmt.conf
project.git = true
project.baseBranch = master
$ scalafmt --diff
... # writes to file

Use cases:

• Interactive IDE setting in a codebase that's not formatted with scalafmt. You could reformat a subsection you edited.
• (hypothetical) gradually introduce scalafmt into a project, or allow a subset of developers to use scalafmt in a codebase (caveat below)
• faster formatting (although nice, I wouldn't call it a selling feature unless for maybe huge source files).

Caveats:

• in a partially formatted codebase, scalafmt may still introduce unnecessarily large diffs. Example

// line limit    |
-  class Foo extends A with B with C
+  class Foo extends A with B with C with D
// after formatting
class Foo
    extends A
    with B
    with C
    with D

A blank enforcement of diff formatting might therefore be unwanted.

Challenges:

• need to guarantee that formatting a sequence of diffs will always produce the same output as formatting everything all the time.

Opening this PR to collect feedback. I would like to get this feature ready for 0.5.

TODOs:

• handle indent for Case
• handle indent for Term.Lambda
• setup test that goes through git repo history and formats diff on every commit. Assert the output is identical to formatting the entire project on every commit.
• select chains
• large config style lists

[olafurpg]

This is blocked by https://github.com/olafurpg/scalafmt/issues/140

[olafurpg]

0.5 made big progress on config argument lists, so this PR is somewhat closer to being ready. However, I'm a bit busy with other tasks at the moment so I'm gonna close this PR.