Currently, git show | arc diff --raw and similar doesn't work because we try to figure out what the "Branch: feature (branched from whatever)" value is, which doesn't make sense.
$ git show | arc diff --raw --trace ARGV '/Users/epriestley/dev/core/lib/arcanist/bin/../scripts/arcanist.php' 'diff' '--raw' '--trace' LOAD Loaded "phutil" from "/Users/epriestley/dev/core/lib/libphutil/src". LOAD Loaded "arcanist" from "/Users/epriestley/dev/core/lib/arcanist/src". Config: Reading user configuration file "/Users/epriestley/.arcrc"... Config: Did not find system configuration at "/etc/arcconfig". Working Copy: Reading .arcconfig from "/Users/epriestley/dev/core/lib/arcanist/.arcconfig". Working Copy: Path "/Users/epriestley/dev/core/lib/arcanist" is part of `git` working copy "/Users/epriestley/dev/core/lib/arcanist". Working Copy: Project root is at "/Users/epriestley/dev/core/lib/arcanist". Config: Reading local configuration file "/Users/epriestley/dev/core/lib/arcanist/.git/arc/config"... Loading phutil library from '/Users/epriestley/dev/core/lib/arcanist/src'... >>> [0] <conduit> conduit.connect() <bytes = 489> >>> [1] <http> https://secure.phabricator.com/api/conduit.connect <<< [1] <http> 211,217 us <<< [0] <conduit> 212,001 us >>> [2] <event> diff.didCollectChanges <listeners = 0> <<< [2] <event> 140 us >>> [3] <event> diff.didBuildMessage <listeners = 0> <<< [3] <event> 46 us Reading diff from stdin... >>> [4] <conduit> differential.creatediff() <bytes = 10,542> >>> [5] <http> https://secure.phabricator.com/api/differential.creatediff <<< [5] <http> 120,215 us <<< [4] <conduit> 120,411 us >>> [6] <event> diff.wasCreated <listeners = 0> <<< [6] <event> 41 us SKIP STAGING Raw changes can not be pushed to a staging area. >>> [7] <conduit> harbormaster.queryautotargets() <bytes = 290> >>> [8] <http> https://secure.phabricator.com/api/harbormaster.queryautotargets <<< [8] <http> 217,717 us <<< [7] <conduit> 217,944 us >>> [9] <conduit> harbormaster.sendmessage() <bytes = 274> >>> [10] <http> https://secure.phabricator.com/api/harbormaster.sendmessage >>> [11] <conduit> harbormaster.sendmessage() <bytes = 274> >>> [12] <http> https://secure.phabricator.com/api/harbormaster.sendmessage <<< [10] <http> 123,821 us <<< [9] <conduit> 134,329 us <<< [12] <http> 227,580 us <<< [11] <conduit> 227,787 us [2016-01-05 10:08:58] EXCEPTION: (Exception) This workflow ('ArcanistDiffWorkflow') requires a Repository API, override requiresRepositoryAPI() to return true. at [<arcanist>/src/workflow/ArcanistWorkflow.php:804] arcanist(head=master, ref.master=b3e68c9f1793), phutil(head=stable, ref.master=adb8a9c074ba, ref.stable=7b8d38cd2d4e) #0 ArcanistWorkflow::getRepositoryAPI() called at [<arcanist>/src/workflow/ArcanistDiffWorkflow.php:2421] #1 ArcanistDiffWorkflow::getDiffOntoTargets() called at [<arcanist>/src/workflow/ArcanistDiffWorkflow.php:2411] #2 ArcanistDiffWorkflow::updateOntoDiffProperty() called at [<arcanist>/src/workflow/ArcanistDiffWorkflow.php:534] #3 ArcanistDiffWorkflow::run() called at [<arcanist>/scripts/arcanist.php:392]