include current Git HEAD in debug output

gallery_dl/__init__.py

@@ -238,8 +238,24 @@ def main():
             config.set(("output", "mode"), "null")
         elif args.loglevel <= logging.DEBUG:
             import platform
+            import subprocess
+            import os.path
             import requests
-            log.debug("Version %s", __version__)
+
+            head = ""
+            try:
+                out, err = subprocess.Popen(
+                    ("git",  "rev-parse", "--short", "HEAD"),
+                    stdout=subprocess.PIPE,
+                    stderr=subprocess.PIPE,
+                    cwd=os.path.dirname(os.path.abspath(__file__)),
+                ).communicate()
+                if out and not err:
+                    head = " - Git HEAD: " + out.decode().rstrip()
+            except (OSError, subprocess.SubprocessError):
+                pass
+
+            log.debug("Version %s%s", __version__, head)
             log.debug("Python %s - %s",
                       platform.python_version(), platform.platform())
             try: