Improve performance of mercurial arc diff

Summary:
arc diff on large mercurial repos was taking 14 seconds just to get
to the commit message prompt.  With these optimizations it takes 4.

- "ancestor(.) - ancestor(XYZ)" is expensive because it has to build the
entire 400000+ revision history for both.  "XYZ::. - XYZ" is much cheaper
because it only looks at the revisions between XYZ and the working directory.

- "hg outgoing" has to talk to the server, which is slow.  "hg log -r draft()"
gives us the same information and is much cheaper.  We fall back to 'outgoing'
on older versions of mercurial.

Of the remaining 4 seconds, 2.5 are spent in 'hg status', which is a bit harder
improve.

Test Plan: Ran arc diff on our hg repo. Verified it ran faster and the diff was created.

Reviewers: epriestley, sid0, bos, dschleimer

Reviewed By: epriestley

CC: aran, Korvin, tido

Differential Revision: https://secure.phabricator.com/D4838

src/repository/api/ArcanistMercurialAPI.php

@@ -106,9 +106,20 @@ final class ArcanistMercurialAPI extends ArcanistRepositoryAPI {
       return $base;
     }
 
-    list($err, $stdout) = $this->execManualLocal(
+    // Mercurial 2.1 and up have phases which indicate if something is
-      'outgoing --branch %s --style default',
+    // published or not. To find which revs are outgoing, it's much
+    // faster to check the phase instead of actually checking the server.
+    list($err) = $this->execManualLocal('help phase');
+    if (!$err) {
+      list($err, $stdout) = $this->execManualLocal(
+        'log --branch %s -r %s --style default',
+        $this->getBranchName(),
+        'draft()');
+    } else {
+      list($err, $stdout) = $this->execManualLocal(
+        'outgoing --branch %s --style default',
       $this->getBranchName());
+    }
 
     if (!$err) {
       $logs = ArcanistMercurialParser::parseMercurialLog($stdout);
@@ -175,13 +186,12 @@ final class ArcanistMercurialAPI extends ArcanistRepositoryAPI {
 
   public function getLocalCommitInformation() {
     if ($this->localCommitInfo === null) {
+      $base_commit = $this->getBaseCommit();
       list($info) = $this->execxLocal(
         "log --template '%C' --rev %s --branch %s --",
         "{node}\1{rev}\1{author|emailuser}\1{author|email}\1".
           "{date|rfc822date}\1{branch}\1{tag}\1{parents}\1{desc}\2",
-        hgsprintf(
+        hgsprintf('(%s::. - %s)', $base_commit, $base_commit),
-          '(ancestors(.) - ancestors(%s))',
-          $this->getBaseCommit()),
         $this->getBranchName());
       $logs = array_filter(explode("\2", $info));
 
@@ -493,9 +503,10 @@ final class ArcanistMercurialAPI extends ArcanistRepositoryAPI {
   }
 
   public function getCommitMessageLog() {
+    $base_commit = $this->getBaseCommit();
     list($stdout) = $this->execxLocal(
       "log --template '{node}\\2{desc}\\1' --rev %s --branch %s --",
-      'ancestors(.) - ancestors('.$this->getBaseCommit().')',
+      hgsprintf('(%s::. - %s)', $base_commit, $base_commit),
       $this->getBranchName());
 
     $map = array();