git-merge: add --ff and --no-ff options

These new options can be used to control the policy for fast-forward
merges: --ff allows it (this is the default) while --no-ff will create
a merge commit.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/merge-options.txt

@@ -27,6 +27,15 @@
 	Perform the merge and commit the result. This option can
 	be used to override --squash.
 
+--no-ff::
+	Generate a merge commit even if the merge resolved as a
+	fast-forward.
+
+--ff::
+	Do not generate a merge commit if the merge resolved as
+	a fast-forward, only update the branch pointer. This is
+	the default behavior of git-merge.
+
 -s <strategy>, \--strategy=<strategy>::
 	Use the given merge strategy; can be supplied more than
 	once to specify them in the order they should be tried.

git-merge.sh

@@ -3,7 +3,7 @@
 # Copyright (c) 2005 Junio C Hamano
 #
 
-USAGE='[-n] [--summary] [--[no-]commit] [--[no-]squash] [-s <strategy>] [-m=<merge-message>] <commit>+'
+USAGE='[-n] [--summary] [--[no-]commit] [--[no-]squash] [--[no-]ff] [-s <strategy>] [-m=<merge-message>] <commit>+'
 
 SUBDIRECTORY_OK=Yes
 . git-sh-setup
@@ -127,13 +127,17 @@ parse_option () {
 	--summary)
 		show_diffstat=t ;;
 	--sq|--squ|--squa|--squas|--squash)
-		squash=t no_commit=t ;;
+		allow_fast_forward=t squash=t no_commit=t ;;
 	--no-sq|--no-squ|--no-squa|--no-squas|--no-squash)
-		squash= no_commit= ;;
+		allow_fast_forward=t squash= no_commit= ;;
 	--c|--co|--com|--comm|--commi|--commit)
-		squash= no_commit= ;;
+		allow_fast_forward=t squash= no_commit= ;;
 	--no-c|--no-co|--no-com|--no-comm|--no-commi|--no-commit)
-		squash= no_commit=t ;;
+		allow_fast_forward=t squash= no_commit=t ;;
+	--ff)
+		allow_fast_forward=t squash= no_commit= ;;
+	--no-ff)
+		allow_fast_forward=false squash= no_commit= ;;
 	-s=*|--s=*|--st=*|--str=*|--stra=*|--strat=*|--strate=*|\
 		--strateg=*|--strategy=*|\
 	-s|--s|--st|--str|--stra|--strat|--strate|--strateg|--strategy)
@@ -477,7 +481,13 @@ done
 # auto resolved the merge cleanly.
 if test '' != "$result_tree"
 then
-    parents=$(git show-branch --independent "$head" "$@" | sed -e 's/^/-p /')
+    if test "$allow_fast_forward" = "t"
+    then
+        parents=$(git show-branch --independent "$head" "$@")
+    else
+        parents=$(git rev-parse "$head" "$@")
+    fi
+    parents=$(echo "$parents" | sed -e 's/^/-p /')
     result_commit=$(printf '%s\n' "$merge_msg" | git commit-tree $result_tree $parents) || exit
     finish "$result_commit" "Merge made by $wt_strategy."
     dropsave

t/t7600-merge.sh

@@ -417,4 +417,24 @@ test_expect_success 'merge c1 with c2 (override --squash)' '
 
 test_debug 'gitk --all'
 
+test_expect_success 'merge c0 with c1 (no-ff)' '
+	git reset --hard c0 &&
+	test_tick &&
+	git merge --no-ff c1 &&
+	verify_merge file result.1 &&
+	verify_parents $c0 $c1
+'
+
+test_debug 'gitk --all'
+
+test_expect_success 'merge c0 with c1 (ff overrides no-ff)' '
+	git reset --hard c0 &&
+	git config branch.master.mergeoptions "--no-ff" &&
+	git merge --ff c1 &&
+	verify_merge file result.1 &&
+	verify_head $c1
+'
+
+test_debug 'gitk --all'
+
 test_done