pull: make pull.ff=true override merge.ff

Since b814da8 (pull: add pull.ff configuration, 2014-01-15), running
git-pull with the configuration pull.ff=false or pull.ff=only is
equivalent to passing --no-ff and --ff-only to git-merge. However, if
pull.ff=true, no switch is passed to git-merge. This leads to the
confusing behavior where pull.ff=false or pull.ff=only is able to
override merge.ff, while pull.ff=true is unable to.

Fix this by adding the --ff switch if pull.ff=true, and add a test to
catch future regressions.

Furthermore, clarify in the documentation that pull.ff overrides
merge.ff.

Signed-off-by: Paul Tan <pyokagan@gmail.com>
Reviewed-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -2033,7 +2033,7 @@ pull.ff::
 	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).
+	command line). This setting overrides `merge.ff` when pulling.
 
 pull.rebase::
 	When true, rebase branches on top of the fetched branch, instead

git-pull.sh

@@ -56,6 +56,9 @@ fi
 # Setup default fast-forward options via `pull.ff`
 pull_ff=$(git config pull.ff)
 case "$pull_ff" in
+true)
+	no_ff=--ff
+	;;
 false)
 	no_ff=--no-ff
 	;;

t/t7601-merge-pull-config.sh

@@ -45,6 +45,14 @@ test_expect_success 'fast-forward pull succeeds with "true" in pull.ff' '
 	test "$(git rev-parse HEAD)" = "$(git rev-parse c1)"
 '
 
+test_expect_success 'pull.ff=true overrides merge.ff=false' '
+	git reset --hard c0 &&
+	test_config merge.ff false &&
+	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 &&