git-merge: do up-to-date check also for all strategies

This clarifies the logic to omit fast-forward check and omit
trivial merge before running the specified strategy.

The "index_merge" variable started out as a flag to say "do not
do anything clever", but when recursive was changed to skip the
trivial merge, the semantics were changed and the variable alone
does not make sense anymore.

This splits the variable into two, allow_fast_forward (which is
almost always true, and avoids making a merge commit when the
other commit is a descendant of our branch, but is set to false
for ours and subtree) and allow_trivial_merge (which is false
for ours, recursive and subtree).

Unlike the earlier implementation, the "ours" strategy allows an
up-to-date condition.  When we are up-to-date, the result will
be our commit, and by definition, we will have our tree as the
result.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-merge.sh

@@ -19,10 +19,12 @@ LF='
 all_strategies='recur recursive octopus resolve stupid ours subtree'
 default_twohead_strategies='recursive'
 default_octopus_strategies='octopus'
-no_trivial_merge_strategies='ours subtree'
+no_fast_forward_strategies='subtree ours'
+no_trivial_strategies='recursive recur subtree ours'
 use_strategies=
 
-index_merge=t
+allow_fast_forward=t
+allow_trivial_merge=t
 
 dropsave() {
 	rm -f -- "$GIT_DIR/MERGE_HEAD" "$GIT_DIR/MERGE_MSG" \
@@ -265,11 +267,20 @@ esac
 
 for s in $use_strategies
 do
-	for nt in $no_trivial_merge_strategies
+	for ss in $no_fast_forward_strategies
 	do
 		case " $s " in
-		*" $nt "*)
-			index_merge=f
+		*" $ss "*)
+			allow_fast_forward=f
+			break
+			;;
+		esac
+	done
+	for ss in $no_trivial_strategies
+	do
+		case " $s " in
+		*" $ss "*)
+			allow_trivial_merge=f
 			break
 			;;
 		esac
@@ -286,10 +297,7 @@ case "$#" in
 esac
 echo "$head" >"$GIT_DIR/ORIG_HEAD"
 
-case "$index_merge,$#,$common,$no_commit" in
-f,*)
-	# We've been told not to try anything clever.  Skip to real merge.
-	;;
+case "$allow_fast_forward,$#,$common,$no_commit" in
 ?,*,'',*)
 	# No common ancestors found. We need a real merge.
 	;;
@@ -299,7 +307,7 @@ f,*)
 	finish_up_to_date "Already up-to-date."
 	exit 0
 	;;
-?,1,"$head",*)
+t,1,"$head",*)
 	# Again the most common case of merging one remote.
 	echo "Updating $(git rev-parse --short $head)..$(git rev-parse --short $1)"
 	git update-index --refresh 2>/dev/null
@@ -322,11 +330,8 @@ f,*)
 	# We are not doing octopus, not fast forward, and have only
 	# one common.
 	git update-index --refresh 2>/dev/null
-	case " $use_strategies " in
-	*' recursive '*|*' recur '*)
-		: run merge later
-		;;
-	*)
+	case "$allow_trivial_merge" in
+	t)
 		# See if it is really trivial.
 		git var GIT_COMMITTER_IDENT >/dev/null || exit
 		echo "Trying really trivial in-index merge..."

t/t6028-merge-up-to-date.sh

@@ -0,0 +1,77 @@
+#!/bin/sh
+
+test_description='merge fast forward and up to date'
+
+. ./test-lib.sh
+
+test_expect_success setup '
+	>file &&
+	git add file &&
+	test_tick &&
+	git commit -m initial &&
+	git tag c0 &&
+
+	echo second >file &&
+	git add file &&
+	test_tick &&
+	git commit -m second &&
+	git tag c1 &&
+	git branch test
+'
+
+test_expect_success 'merge -s recursive up-to-date' '
+
+	git reset --hard c1 &&
+	test_tick &&
+	git merge -s recursive c0 &&
+	expect=$(git rev-parse c1) &&
+	current=$(git rev-parse HEAD) &&
+	test "$expect" = "$current"
+
+'
+
+test_expect_success 'merge -s recursive fast-forward' '
+
+	git reset --hard c0 &&
+	test_tick &&
+	git merge -s recursive c1 &&
+	expect=$(git rev-parse c1) &&
+	current=$(git rev-parse HEAD) &&
+	test "$expect" = "$current"
+
+'
+
+test_expect_success 'merge -s ours up-to-date' '
+
+	git reset --hard c1 &&
+	test_tick &&
+	git merge -s ours c0 &&
+	expect=$(git rev-parse c1) &&
+	current=$(git rev-parse HEAD) &&
+	test "$expect" = "$current"
+
+'
+
+test_expect_success 'merge -s ours fast-forward' '
+
+	git reset --hard c0 &&
+	test_tick &&
+	git merge -s ours c1 &&
+	expect=$(git rev-parse c0^{tree}) &&
+	current=$(git rev-parse HEAD^{tree}) &&
+	test "$expect" = "$current"
+
+'
+
+test_expect_success 'merge -s subtree up-to-date' '
+
+	git reset --hard c1 &&
+	test_tick &&
+	git merge -s subtree c0 &&
+	expect=$(git rev-parse c1) &&
+	current=$(git rev-parse HEAD) &&
+	test "$expect" = "$current"
+
+'
+
+test_done