dim: Handle fast-forwards separately in update_nightly

We've had tons of fun in the past with -nightly merge commits, e.g.

commit e47410aa
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Mon Sep 8 08:52:41 2014 +0200

    dim: cope better with silent conflict fixup patches

Now another kind of fun showed up, namely when a merge is just a
fast-forward without any content changes. That lead to the git diff
--cached check failing, which lead to not committing the merge, which
lead to to all subsequent merges failing too.

Since this is all too fragile rework the logic a bit and first filter
out the fast-forward merges. With those handled we know that there
will be a real merge and so always something to commit. Which also
means that we'll never have fun again with fixup patches getting lost.
Hopefully.

dim

@@ -223,6 +223,9 @@ function update_nightly
 		if [ $first == 1 ] ; then
 			git reset --hard $sha1 >& /dev/null
 			first=0
+		elif git merge --rerere-autoupdate --ff-only $sha1 >& /dev/null ; then
+			# nothing to do if just fast-forward
+			true
 		else
 			local fixup_file=$rerere/$integration_branch-$name-fixup.patch
 
@@ -236,14 +239,9 @@ function update_nightly
 			check_conflicts
 			git add -u
 
-			if [[ `git diff --cached --name-status | wc -l` -gt 0 ]]; then
-				# If the conflict is silent git merge succeeded,
-				# so there's no cached commit message around.
-				# Since it's too hard to detect this reliably
-				# (git scripting is fragile) just try a second
-				# time with a commit message supplied.
-				git commit --no-edit || git commit -m "Applying $fixup_file"
-			fi
+			# because we filter out fast-forward merges there will
+			# always be something to commit
+			git commit --no-edit
 		fi
 
 		echo -e "$branch `git rev-parse $sha1`\n\t`git log -1 $sha1 --pretty=format:%s`" >> $specfile