Add an 'errno' property to the Error object in callbacks, and set it …

…to 'process.ENOENT' when a "Path 'path' does not exist in 'commit-ref'" error occurs.

lib/git-fs.js

@@ -29,6 +29,8 @@ var CACHE_LIFE = [36300000, 100];
 
 var gitCommands, gitDir, workTree;
 
+var gitENOENT = /fatal: Path '([^']+)' does not exist in '([0-9a-f]{40})'/;
+
 // Set up the git configs for the subprocess
 var Git = module.exports = function (repo) {
   // Check the directory exists first.
@@ -173,7 +175,11 @@ function gitExec(commands, callback) {
   });
   child.addListener('exit', function (code) {
     if (code > 0) {
-      callback(new Error("git " + commands.join(" ") + "\n" + stderr.join('')));
+      var err = new Error("git " + commands.join(" ") + "\n" + stderr.join(''));
+      if (gitENOENT.test(err.message)) {
+        err.errno = process.ENOENT;
+      }
+      callback(err);
       return;
     }
     callback(null, stdout.join(''));