pull: add pull.ff configuration

Add a `pull.ff` configuration option that is analogous
to the `merge.ff` option.

This allows us to control the fast-forward behavior for
pull-initiated merges only.

Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -1877,6 +1877,16 @@ pretty.<name>::
 	Note that an alias with the same name as a built-in format
 	will be silently ignored.
 
+pull.ff::
+	By default, Git does not create an extra merge commit when merging
+	a commit that is a descendant of the current commit. Instead, the
+	tip of the current branch is fast-forwarded. When set to `false`,
+	this variable tells Git to create an extra merge commit in such
+	a case (equivalent to giving the `--no-ff` option from the command
+	line). When set to `only`, only such fast-forward merges are
+	allowed (equivalent to giving the `--ff-only` option from the
+	command line).
+
 pull.rebase::
 	When true, rebase branches on top of the fetched branch, instead
 	of merging the default branch from the default remote when "git

git-pull.sh

@@ -52,6 +52,21 @@ if test -z "$rebase"
 then
 	rebase=$(bool_or_string_config pull.rebase)
 fi
+
+# Setup default fast-forward options via `pull.ff`
+pull_ff=$(git config pull.ff)
+case "$pull_ff" in
+false)
+	no_ff=--no-ff
+	break
+	;;
+only)
+	ff_only=--ff-only
+	break
+	;;
+esac
+
+
 dry_run=
 while :
 do

t/t7601-merge-pull-config.sh

@@ -38,6 +38,27 @@ test_expect_success 'merge c1 with c2' '
 	test -f c2.c
 '
 
+test_expect_success 'fast-forward pull succeeds with "true" in pull.ff' '
+	git reset --hard c0 &&
+	test_config pull.ff true &&
+	git pull . c1 &&
+	test "$(git rev-parse HEAD)" = "$(git rev-parse c1)"
+'
+
+test_expect_success 'fast-forward pull creates merge with "false" in pull.ff' '
+	git reset --hard c0 &&
+	test_config pull.ff false &&
+	git pull . c1 &&
+	test "$(git rev-parse HEAD^1)" = "$(git rev-parse c0)" &&
+	test "$(git rev-parse HEAD^2)" = "$(git rev-parse c1)"
+'
+
+test_expect_success 'pull prevents non-fast-forward with "only" in pull.ff' '
+	git reset --hard c1 &&
+	test_config pull.ff only &&
+	test_must_fail git pull . c3
+'
+
 test_expect_success 'merge c1 with c2 (ours in pull.twohead)' '
 	git reset --hard c1 &&
 	git config pull.twohead ours &&