Currently, `arc branch` fails when not on a branch (for example, when detached from a branch for whatever reason).
```
> git checkout facebook/master
> arc --trace branch
libphutil loaded from '/home/joshua/dotfiles/modules/phabricator/libphutil/src'.
arcanist loaded from '/home/joshua/dotfiles/modules/phabricator/arcanist/src'.
Working Copy: Reading .arcconfig from "/home/joshua/workspace/github.com/facebook/arcanist/.arcconfig".
Working Copy: Path "/home/joshua/workspace/github.com/facebook/arcanist" is part of `git` working copy "/home/joshua/workspace/github.com/facebook/arcanist".
Working Copy: Project root is at "/home/joshua/workspace/github.com/facebook/arcanist".
This is arcanist! Forcing this copy to run...
>>> [0] <exec> $ ARC_PHUTIL_PATH='/home/joshua/dotfiles/modules/phabricator/libphutil' '/home/joshua/workspace/github.com/facebook/arcanist/bin/arc' '--trace' 'branch'
libphutil loaded from '/home/joshua/dotfiles/modules/phabricator/libphutil/src'.
arcanist loaded from '/home/joshua/workspace/github.com/facebook/arcanist/src'.
Working Copy: Reading .arcconfig from "/home/joshua/workspace/github.com/facebook/arcanist/.arcconfig".
Working Copy: Path "/home/joshua/workspace/github.com/facebook/arcanist" is part of `git` working copy "/home/joshua/workspace/github.com/facebook/arcanist".
Working Copy: Project root is at "/home/joshua/workspace/github.com/facebook/arcanist".
Loading phutil library from '/home/joshua/workspace/github.com/facebook/arcanist/src'...
>>> [0] <conduit> conduit.connect() <bytes = 517>
>>> [1] <http> https://secure.phabricator.com/api/conduit.connect
<<< [1] <http> 1,511,623 us
<<< [0] <conduit> 1,512,006 us
>>> [2] <exec> $ git branch --no-color
<<< [2] <exec> 5,213 us
>>> [3] <exec> $ git show -s --format=%H%x01%ct%x01%T%x01%s%x01%s%n%n%b '(detached from facebook/master)' --
>>> [4] <exec> $ git show -s --format=%H%x01%ct%x01%T%x01%s%x01%s%n%n%b 'arcpatch-D4519' --
>>> [5] <exec> $ git show -s --format=%H%x01%ct%x01%T%x01%s%x01%s%n%n%b 'arcpatch-D6810' --
>>> [6] <exec> $ git show -s --format=%H%x01%ct%x01%T%x01%s%x01%s%n%n%b 'configuration' --
>>> [7] <exec> $ git show -s --format=%H%x01%ct%x01%T%x01%s%x01%s%n%n%b 'docblock_comments' --
>>> [8] <exec> $ git show -s --format=%H%x01%ct%x01%T%x01%s%x01%s%n%n%b 'external_linter_version' --
>>> [9] <exec> $ git show -s --format=%H%x01%ct%x01%T%x01%s%x01%s%n%n%b 'flags_as_array' --
>>> [10] <exec> $ git show -s --format=%H%x01%ct%x01%T%x01%s%x01%s%n%n%b 'lint_engine-final_methods' --
>>> [11] <exec> $ git show -s --format=%H%x01%ct%x01%T%x01%s%x01%s%n%n%b 'linter-final_methods' --
>>> [12] <exec> $ git show -s --format=%H%x01%ct%x01%T%x01%s%x01%s%n%n%b 'linter-test_method' --
>>> [13] <exec> $ git show -s --format=%H%x01%ct%x01%T%x01%s%x01%s%n%n%b 'master' --
>>> [14] <exec> $ git show -s --format=%H%x01%ct%x01%T%x01%s%x01%s%n%n%b 'spelling_data' --
>>> [15] <exec> $ git show -s --format=%H%x01%ct%x01%T%x01%s%x01%s%n%n%b 'tidying-linters' --
>>> [16] <exec> $ git show -s --format=%H%x01%ct%x01%T%x01%s%x01%s%n%n%b 'workflow-final_methods' --
>>> [17] <exec> $ git show -s --format=%H%x01%ct%x01%T%x01%s%x01%s%n%n%b 'xhpast-array_trailing_comma' --
>>> [18] <exec> $ git show -s --format=%H%x01%ct%x01%T%x01%s%x01%s%n%n%b 'xhpast-double_quote' --
<<< [3] <exec> 25,476 us
<<< [4] <exec> 24,607 us
<<< [5] <exec> 23,294 us
<<< [6] <exec> 21,626 us
<<< [7] <exec> 17,563 us
<<< [8] <exec> 15,295 us
<<< [9] <exec> 13,203 us
<<< [10] <exec> 11,836 us
<<< [11] <exec> 10,664 us
<<< [12] <exec> 10,935 us
<<< [13] <exec> 10,370 us
<<< [14] <exec> 9,613 us
<<< [15] <exec> 8,713 us
<<< [16] <exec> 10,060 us
<<< [17] <exec> 10,358 us
<<< [18] <exec> 9,566 us
[2014-03-05 21:13:23] EXCEPTION: (CommandException) Command failed with error #128!
COMMAND
git show -s --format=%H%x01%ct%x01%T%x01%s%x01%s%n%n%b '(detached from facebook/master)' --
STDOUT
(empty)
STDERR
fatal: bad revision '(detached from facebook/master)'
at [/home/joshua/dotfiles/modules/phabricator/libphutil/src/future/exec/ExecFuture.php:398]
#0 ExecFuture::resolvex() called at [/home/joshua/workspace/github.com/facebook/arcanist/src/workflow/ArcanistFeatureWorkflow.php:207]
#1 ArcanistFeatureWorkflow::loadCommitInfo(Array of size 16 starting with: { 0 => Array of size 2 starting with: { current => true } }) called at [/home/joshua/workspace/github.com/facebook/arcanist/src/workflow/ArcanistFeatureWorkflow.php:95]
#2 ArcanistFeatureWorkflow::run() called at [/home/joshua/workspace/github.com/facebook/arcanist/src/workflow/ArcanistBranchWorkflow.php:36]
#3 ArcanistBranchWorkflow::run() called at [/home/joshua/workspace/github.com/facebook/arcanist/scripts/arcanist.php:321]
<<< [0] <exec> 1,638,684 us
```