Add ArcanistLintSeverity::SEVERITY_AUTOFIX.

Summary:
Xcode (a popular code editor on Mac OS X) has no facility
to trim trailing whitespace automatically.

This adds a new lint severity "AUTOFIX" that's between
WARNING and ERROR. When running the linter, any lint message
whose severity is AUTOFIX will automatically be patched.

Furthermore, if all lint messages returned from the engine are
AUTOFIX, we'll automatically amend HEAD with the patch.

Test Plan:
arc lint on files with and without trailing whitespace,
with and without UTF-8 contents to confirm those still error

Reviewers: epriestley, jungejason

Reviewed By: epriestley

CC: aran

Differential Revision: https://secure.phabricator.com/D2125
Allow defining aliases in .arcconfig.

Summary:
For Objective-C repositories, we want to provide aliases to
arc diff --amend-autofixes by default.

This adds the ability to define aliases in .arcconfig (overridden
by any specified in the user config, of course).

Test Plan:
Tested defining alias with nothing in .arcconfig, with
an alias in .arcconfig.  Tested arc alias outside of working
repository.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran

Differential Revision: https://secure.phabricator.com/D2191
If a Git upstream is configured for the current branch, always use that as the default relative commit

Summary: See discussion in D1861.

Test Plan: Ran "arc diff" on master, got an upstream-based relative commit. Ran "arc diff" on a feature branch, got a config-based relative commit. Ran "arc diff x", got an argument-based relative commit.

Reviewers: btrahan, vrana, davidreuss, elgenie

Reviewed By: davidreuss

CC: aran

Differential Revision: https://secure.phabricator.com/D2192
Don't let "arc which" identify the working copy as belonging to a different project

Summary: If you have two projects (say, libphutil and arcanist) and you prepare a patch for one of them on branch "master", run "arc diff", and then prepare a patch for the other one on the same branch, "arc diff" will try to update the first revision when you run it. Instead, make it smart enough to stay within arc projects.

Test Plan: Ran "arc which" in circumstances where it previously generated a false positive, no false positive.

Reviewers: btrahan, vrana, jungejason

Reviewed By: jungejason

CC: aran

Maniphest Tasks: T1100

Differential Revision: https://secure.phabricator.com/D2199
Read default relative commit from scratch file with newline

Summary:
When people create the .arc/default-relative-commit scratchfile with $EDITOR of
choice, their editor usually puts a newline at the end, which breaks arc diff.
We should trim the newline before using the contents of the scratchfile.

Test Plan:
ran arc diff in a working copy that contained a .arc/default-relative-commit
with a newline

Reviewers: epriestley, jungejason

Reviewed By: epriestley

CC: aran

Differential Revision: https://secure.phabricator.com/D2209
Fix some --only / --preview / SVN issues with Arcanist

Summary:
  - Historically, "--preview" was forbidden under SVN. No reason for that now.
  - The "--auto" patch moved the "--preview" / "--only" checks later than they should be.
  - Fix an issue with Conduit query construction in SVN.

Test Plan: Ran "arc diff --preview" in an SVN working copy. Ran "arc diff" in an SVN working copy.

Reviewers: svemir, btrahan, vrana, jungejason

Reviewed By: svemir

CC: aran

Differential Revision: https://secure.phabricator.com/D2218
Merge remote-tracking branch 'upstream/master'