merge: seed the commit message with list of conflicted files.

The files with conflicts need to be hand resolved, and it is a
good discipline for the committer to explain which branch was
taken and why.  Pre-fill the merge message template with the
list of conflicted paths to encourage it.

This is from Linus.

Signed-off-by: Junio C Hamano <junkio@cox.net>

git-merge.sh

@@ -301,5 +301,13 @@ then
 	"Automatic merge went well; stopped before committing as requested"
 	exit 0
 else
+	{
+	    echo '
+Conflicts:
+'
+		git ls-files --unmerged |
+		sed -e 's/^[^	]*	/	/' |
+		uniq
+	} >>"$GIT_DIR/MERGE_MSG"
 	die "Automatic merge failed; fix up by hand"
 fi