diff --git a/repo.php b/repo.php
index 2bbec439..bcbe0be4 100644
--- a/repo.php
+++ b/repo.php
@@ -34,6 +34,8 @@ if (!isset($_REQUEST['repo'])) {
 }
 
 $repoId = $_REQUEST['repo'];
+list($repoId, $branchId) = explode('/', $repoId);
+
 if (!array_key_exists($repoId, $allRepositories)) {
   Header("HTTP/1.1 404 Repository not found");
   exit(0);
@@ -42,6 +44,16 @@ if (!array_key_exists($repoId, $allRepositories)) {
 $repoData = $allRepositories[$repoId];
 $repo = getRepo($repoId, $repoData);
 
+if ($branchId) {
+  try {
+    $repo->setBranch($branchId);
+  }
+  catch (Exception $e) {
+    Header("HTTP/1.1 404 No such branch");
+    exit(0);
+  }
+}
+
 $cacheDir = null;
 $ts = $repo->timestamp($path);
 if (isset($config['cache'])) {
diff --git a/src/RepositoryGit.php b/src/RepositoryGit.php
index b0e6f4d6..d35ff30b 100644
--- a/src/RepositoryGit.php
+++ b/src/RepositoryGit.php
@@ -2,10 +2,27 @@
 class RepositoryGit extends RepositoryBase {
   function __construct ($id, $def) {
     parent::__construct($id, $def);
-    $this->branch = $def['branch'] ?? 'HEAD';
+    if (array_key_exists('branch', $def)) {
+      $this->branch = $def['branch'];
+    }
+    else {
+      $this->branch = chop(shell_exec("cd " . escapeShellArg($this->path) . "; git rev-parse --abbrev-ref HEAD"));
+    }
+
     $this->branchEsc = escapeShellArg($this->branch);
   }
 
+  function setBranch ($branch) {
+    $this->branch = $branch;
+    $this->branchEsc = escapeShellArg($this->branch);
+
+    exec("cd " . escapeShellArg($this->path) . "; git ls-tree {$this->branchEsc}", $output, $return);
+
+    if ($return !== 0) {
+      throw new Exception('no such branch');
+    }
+  }
+
   function timestamp () {
     $ts = (int)shell_exec("cd " . escapeShellArg($this->path) . "; git log -1 {$this->branchEsc} --pretty=format:%ct");
 
@@ -40,6 +57,20 @@ class RepositoryGit extends RepositoryBase {
     }
     pclose($d);
 
+    if (!array_key_exists('branch', $this->def)) {
+      $d = popen("cd " . escapeShellArg($this->path) . "; git for-each-ref --sort=-committerdate refs/heads/", "r");
+      $data['branch'] = $this->branch;
+      $data['branches'] = array();
+      while ($r = fgets($d)) {
+        if (preg_match("/^([0-9a-f]{40}) commit\trefs\/heads\/(.*)$/", $r, $m)) {
+          $data['branches'][$m[2]] = array(
+            'commit' => $m[1],
+          );
+        }
+      }
+      pclose($d);
+    }
+
     return $data;
   }