rewrite git-howto for the no merge policy

Originally committed as revision 26400 to svn://svn.ffmpeg.org/ffmpeg/trunk

doc/git-howto.txt

@@ -56,17 +56,23 @@ I. BASICS:
 
 2. Updating the source tree to the latest revision:
 
-    git pull
+    git pull (--ff-only)
 
-  pulls in the latest changes from the repository to your local master branch.
+  pulls in the latest changes from the tracked branch. The tracked branch
+  can be remote. By default the master branch tracks the branch master in
+  the remote origin.
+  Caveat: Since merge commits are forbidden at least for the initial
+          months of git --ff-only or --rebase (see below) are recommended.
+          --ff-only will fail and not create merge commits if your branch
+          has diverged (has a different history) from the tracked branch.
 
 2.a Rebasing your local branches:
 
     git pull --rebase
 
   fetches the changes from the main repository and replays your local commits
-  over it. This is useful to keep all your local changes at the top of your
-  tree.
+  over it. This is required to keep all your local changes at the top of
+  FFmpeg's master tree. The master tree will reject pushes with merge commits.
 
 
 3. Adding/removing files/directories: