Resolve HEAD in ksurl to actual hash

When the image build configuration specifies kickstart URL as a HEAD of
a git repo, pungi now figures out what the actual hash of that commit is
and uses that hash instead. This might make logs clearer and should
prevent potential problems if someone pushes to that repo during
composing.

Documentation is updated to mention this.

Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>

tests/test_util.py

@@ -0,0 +1,50 @@
+#!/usr/bin/env python2
+# -*- coding: utf-8 -*-
+
+import mock
+import os
+import sys
+import unittest
+
+sys.path.insert(0, os.path.join(os.path.dirname(__file__), ".."))
+
+from pungi import util
+
+
+class TestGitRefResolver(unittest.TestCase):
+
+    @mock.patch('pungi.util.run')
+    def test_successful_resolve(self, run):
+        run.return_value = (0, 'CAFEBABE\tHEAD\n')
+
+        url = util.resolve_git_url('https://git.example.com/repo.git?somedir#HEAD')
+
+        self.assertEqual(url, 'https://git.example.com/repo.git?somedir#CAFEBABE')
+        run.assert_called_once_with(['git', 'ls-remote', 'https://git.example.com/repo.git', 'HEAD'])
+
+    @mock.patch('pungi.util.run')
+    def test_resolve_missing_spec(self, run):
+        url = util.resolve_git_url('https://git.example.com/repo.git')
+
+        self.assertEqual(url, 'https://git.example.com/repo.git')
+        self.assertEqual(run.mock_calls, [])
+
+    @mock.patch('pungi.util.run')
+    def test_resolve_non_head_spec(self, run):
+        url = util.resolve_git_url('https://git.example.com/repo.git#some-tag')
+
+        self.assertEqual(url, 'https://git.example.com/repo.git#some-tag')
+        self.assertEqual(run.mock_calls, [])
+
+    @mock.patch('pungi.util.run')
+    def test_resolve_ambiguous(self, run):
+        run.return_value = (0, 'CAFEBABE\tF11\nDEADBEEF\tF10\n')
+
+        with self.assertRaises(RuntimeError):
+            util.resolve_git_url('https://git.example.com/repo.git?somedir#HEAD')
+
+        run.assert_called_once_with(['git', 'ls-remote', 'https://git.example.com/repo.git', 'HEAD'])
+
+
+if __name__ == "__main__":
+    unittest.main()