Allow Diffusion to display the initial commit in Git repositories

Summary: See T507. Since you can't do "xxxxxxxx^" where "xxxxxxxx" is the first
commit in a repository, fall back to diffing against the empty tree if we fail
to diff against the parent commit.

Test Plan: Looked at the first commit in libphutil on my local.

Reviewers: edward, jungejason, nh, tuomaspelkonen, aran

Reviewed By: nh

CC: aran, edward, epriestley, nh

Differential Revision: 953

src/applications/diffusion/query/diff/git/DiffusionGitDiffQuery.php

@@ -44,12 +44,36 @@ final class DiffusionGitDiffQuery extends DiffusionDiffQuery {
     );
     $options = implode(' ', $options);
 
+    try {
       list($raw_diff) = execx(
-      "(cd %s && git diff {$options} %s^ %s -- %s)",
+        "(cd %s && git diff %C %s^ %s -- %s)",
         $repository->getDetail('local-path'),
+        $options,
         $effective_commit,
         $effective_commit,
         $drequest->getPath());
+    } catch (CommandException $ex) {
+      // Check if this is the root commit by seeing if it has parents.
+      list($parents) = execx(
+        '(cd %s && git log --format=%s %s --)',
+        $repository->getDetail('local-path'),
+        '%P', // "parents"
+        $effective_commit);
+      if (!strlen(trim($parents))) {
+        // No parents means we're looking at the root revision. Diff against
+        // the empty tree hash instead, since there is no parent so "^" does
+        // not work. See ArcanistGitAPI for more discussion.
+        list($raw_diff) = execx(
+          '(cd %s && git diff %C %s %s -- %s)',
+          $repository->getDetail('local-path'),
+          $options,
+          ArcanistGitAPI::GIT_MAGIC_ROOT_COMMIT,
+          $effective_commit,
+          $drequest->getPath());
+      } else {
+        throw $ex;
+      }
+    }
 
     $parser = new ArcanistDiffParser();
     $parser->setDetectBinaryFiles(true);

src/applications/diffusion/query/diff/git/__init__.php

@@ -7,6 +7,7 @@
 
 
 phutil_require_module('arcanist', 'parser/diff');
+phutil_require_module('arcanist', 'repository/api/git');
 
 phutil_require_module('phabricator', 'applications/differential/storage/diff');
 phutil_require_module('phabricator', 'applications/diffusion/query/diff/base');

src/applications/repository/controller/edit/PhabricatorRepositoryEditController.php

@@ -405,7 +405,7 @@ class PhabricatorRepositoryEditController
       if ($is_git) {
         $instructions =
           'Enter the URI to clone this repository from. It should look like '.
-          '<tt>git@github.com:example/example.git</tt>, <tt> '.
+          '<tt>git@github.com:example/example.git</tt> or '.
           '<tt>ssh://user@host.com/git/example.git</tt>';
       } else if ($is_mercurial) {
         $instructions =