diff --git a/src/applications/diffusion/query/history/DiffusionMercurialHistoryQuery.php b/src/applications/diffusion/query/history/DiffusionMercurialHistoryQuery.php
index c1aafb5e2e..7b6add1adb 100644
--- a/src/applications/diffusion/query/history/DiffusionMercurialHistoryQuery.php
+++ b/src/applications/diffusion/query/history/DiffusionMercurialHistoryQuery.php
@@ -26,20 +26,36 @@ final class DiffusionMercurialHistoryQuery extends DiffusionHistoryQuery {
     $commit_hash = $drequest->getStableCommitName();
 
     $path = DiffusionPathIDQuery::normalizePath($path);
+    $path = ltrim($path, '/');
 
-    // NOTE: Using '' as a default path produces the correct behavior if HEAD
-    // is a merge commit; using '.' does not (the merge commit is not included
-    // in the log).
-    $default_path = '';
+    // NOTE: Older versions of Mercurial give different results for these
+    // commands (see T1268):
+    //
+    //   $ hg log -- ''
+    //   $ hg log
+    //
+    // All versions of Mercurial give different results for these commands
+    // (merge commits are excluded with the "." version):
+    //
+    //   $ hg log -- .
+    //   $ hg log
+    //
+    // If we don't have a path component in the query, omit it from the command
+    // entirely to avoid these inconsistencies.
+
+    $path_arg = '';
+    if (strlen($path)) {
+      $path_arg = csprintf('-- %s', $path);
+    }
 
     // NOTE: --branch used to be called --only-branch; use -b for compatibility.
     list($stdout) = $repository->execxLocalCommand(
-      'log --debug --template %s --limit %d -b %s --rev %s:0 -- %s',
+      'log --debug --template %s --limit %d -b %s --rev %s:0 %C',
       '{node};{parents}\\n',
       ($this->getOffset() + $this->getLimit()), // No '--skip' in Mercurial.
       $drequest->getBranch(),
       $commit_hash,
-      nonempty(ltrim($path, '/'), $default_path));
+      $path_arg);
 
     $lines = explode("\n", trim($stdout));
     $lines = array_slice($lines, $this->getOffset());
diff --git a/src/applications/diffusion/request/DiffusionMercurialRequest.php b/src/applications/diffusion/request/DiffusionMercurialRequest.php
index 99d6baf720..b2ef171813 100644
--- a/src/applications/diffusion/request/DiffusionMercurialRequest.php
+++ b/src/applications/diffusion/request/DiffusionMercurialRequest.php
@@ -59,8 +59,22 @@ final class DiffusionMercurialRequest extends DiffusionRequest {
       if ($this->commit) {
         $this->stableCommitName = $this->commit;
       } else {
+
+        // NOTE: For branches with spaces in their name like "a b", this
+        // does not work properly:
+        //
+        //   $ hg log --rev 'a b'
+        //
+        // We can use revsets instead:
+        //
+        //   $ hg log --rev branch('a b')
+        //
+        // ...but they require a somewhat newer version of Mercurial. Instead,
+        // use "-b" flag with limit 1 for greatest compatibility across
+        // versions.
+
         list($this->stableCommitName) = $this->repository->execxLocalCommand(
-          'log --template=%s --rev %s',
+          'log --template=%s -b %s --limit 1',
           '{node}',
           $this->getBranch());
       }