Fix update notifications

* Python 2 incompatibility
* Handle actual update, .remote_head_rev needs to be updated
  in that case

mpcdf_common.py

@@ -51,7 +51,7 @@ def check_for_update():
     import os
     import sys
     import time
-    from subprocess import check_output
+    from subprocess import check_output, call
 
     if hasattr(sys.modules["mpcdf_common"], "checked_for_updates"):
         return
@@ -60,25 +60,41 @@ def check_for_update():
 
     plugin_dir = os.path.dirname(os.path.realpath(__file__))
     git_dir = os.path.join(plugin_dir, ".git")
-    local_rev = check_output(["git", "--git-dir", git_dir, "rev-parse", "HEAD"]).strip()
+    local_rev = check_output(["git", "--git-dir", git_dir, "rev-parse", "HEAD"], encoding="utf-8").strip()
+
+    url = "https://gitlab.mpcdf.mpg.de/mpcdf/osc-plugins.git"
+
+    def update_server_rev():
+        server_rev, _ = check_output(["git", "ls-remote", url, "master"], encoding="utf-8").split(maxsplit=1)
+        with open(rev_file, "w") as fd:
+            fd.write(server_rev)
 
     # Check for update on server just once a day
     rev_file = os.path.join(plugin_dir, ".remote_head_rev")
     try:
         mtime = os.stat(rev_file).st_mtime
-    except FileNotFoundError:
+    except EnvironmentError:
         mtime = 0
 
-    url = "https://gitlab.mpcdf.mpg.de/mpcdf/osc-plugins.git"
-
     if time.time() - mtime > 86400:
-        server_rev, _ = check_output(["git", "ls-remote", url, "master"]).split(maxsplit=1)
-        with open(rev_file, "wb") as fd:
-            fd.write(server_rev)
+        update_server_rev()
 
-    with open(rev_file, "rb") as fd:
+    with open(rev_file, "r") as fd:
         server_rev = fd.read().strip()
-        if server_rev != local_rev:
+
+    if call(["git", "--git-dir", git_dir, "merge-base", "--is-ancestor", server_rev, "HEAD"]) == 0:
+        # Server rev is older than ours. Check again
+        update_server_rev()
+
+    with open(rev_file, "r") as fd:
+        server_rev = fd.read().strip()
+
+    if server_rev != local_rev:
+        if call(["git", "--git-dir", git_dir, "merge-base", "--is-ancestor", server_rev, "HEAD"]) == 0:
+            print(file=sys.stderr)
+            print("You have unpushed commits in", plugin_dir, "- consider pushing them", file=sys.stderr)
+            print(file=sys.stderr)
+        else:
             print(file=sys.stderr)
             print("Your plugin directory is out-of-date, new commits available on", url, file=sys.stderr)
             print(file=sys.stderr)