git-merge knows some strategies want to skip trivial merges

Most notably "ours".  Also this makes sure we do not record
duplicated parents on the parent list of the resulting commit.

This is based on Mark Wooding's work, but does not change the UI
nor introduce new flags.

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

git-merge.sh

@@ -11,11 +11,15 @@ LF='
 '
 
 all_strategies='recursive octopus resolve stupid ours'
-default_strategies='recursive'
+default_twohead_strategies='recursive'
+default_octopus_strategies='octopus'
+no_trivial_merge_strategies='ours'
 use_strategies=
+
+index_merge=t
 if test "@@NO_PYTHON@@"; then
 	all_strategies='resolve octopus stupid ours'
-	default_strategies='resolve'
+	default_twohead_strategies='resolve'
 fi
 
 dropsave() {
@@ -90,8 +94,6 @@ do
 	shift
 done
 
-test "$#" -le 2 && usage ;# we need at least two heads.
-
 merge_msg="$1"
 shift
 head_arg="$1"
@@ -99,6 +101,8 @@ head=$(git-rev-parse --verify "$1"^0) || usage
 shift
 
 # All the rest are remote heads
+test "$#" = 0 && usage ;# we need at least one remote head.
+
 remoteheads=
 for remote
 do
@@ -108,6 +112,27 @@ do
 done
 set x $remoteheads ; shift
 
+case "$use_strategies" in
+'')
+	case "$#" in
+	1)
+		use_strategies="$default_twohead_strategies" ;;
+	*)
+		use_strategies="$default_octopus_strategies" ;;
+	esac
+	;;
+esac
+
+for s in $use_strategies
+do
+	case " $s " in
+	*" $no_trivial_merge_strategies "*)
+		index_merge=f
+		break
+		;;
+	esac
+done
+
 case "$#" in
 1)
 	common=$(git-merge-base --all $head "$@")
@@ -118,18 +143,21 @@ case "$#" in
 esac
 echo "$head" >"$GIT_DIR/ORIG_HEAD"
 
-case "$#,$common,$no_commit" in
-*,'',*)
+case "$index_merge,$#,$common,$no_commit" in
+f,*)
+	# We've been told not to try anything clever.  Skip to real merge.
+	;;
+?,*,'',*)
 	# No common ancestors found. We need a real merge.
 	;;
-1,"$1",*)
+?,1,"$1",*)
 	# If head can reach all the merge then we are up to date.
-	# but first the most common case of merging one remote
+	# but first the most common case of merging one remote.
 	echo "Already up-to-date."
 	dropsave
 	exit 0
 	;;
-1,"$head",*)
+?,1,"$head",*)
 	# Again the most common case of merging one remote.
 	echo "Updating from $head to $1"
 	git-update-index --refresh 2>/dev/null
@@ -139,11 +167,11 @@ case "$#,$common,$no_commit" in
 	dropsave
 	exit 0
 	;;
-1,?*"$LF"?*,*)
+?,1,?*"$LF"?*,*)
 	# We are not doing octopus and not fast forward.  Need a
 	# real merge.
 	;;
-1,*,)
+?,1,*,)
 	# We are not doing octopus, not fast forward, and have only
 	# one common.  See if it is really trivial.
 	git var GIT_COMMITTER_IDENT >/dev/null || exit
@@ -188,17 +216,6 @@ esac
 # We are going to make a new commit.
 git var GIT_COMMITTER_IDENT >/dev/null || exit
 
-case "$use_strategies" in
-'')
-	case "$#" in
-	1)
-		use_strategies="$default_strategies" ;;
-	*)
-		use_strategies=octopus ;;
-	esac		
-	;;
-esac
-
 # At this point, we need a real merge.  No matter what strategy
 # we use, it would operate on the index, possibly affecting the
 # working tree, and when resolved cleanly, have the desired tree
@@ -270,11 +287,7 @@ done
 # auto resolved the merge cleanly.
 if test '' != "$result_tree"
 then
-    parents="-p $head"
-    for remote
-    do
-        parents="$parents -p $remote"
-    done
+    parents=$(git-show-branch --independent "$head" "$@" | sed -e 's/^/-p /')
     result_commit=$(echo "$merge_msg" | git-commit-tree $result_tree $parents) || exit
     finish "$result_commit" "Merge $result_commit, made by $wt_strategy."
     dropsave