merge: introduce merge.ff configuration variable

This variable gives the default setting for --ff, --no-ff or --ff-only
options of "git merge" command.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/merge-config.txt

@@ -16,6 +16,16 @@ merge.defaultToUpstream::
 	to their corresponding remote tracking branches, and the tips of
 	these tracking branches are merged.
 
+merge.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).
+
 merge.log::
 	In addition to branch names, populate the log message with at
 	most the specified number of one-line descriptions from the

builtin/merge.c

@@ -550,6 +550,15 @@ static int git_merge_config(const char *k, const char *v, void *cb)
 		if (is_bool && shortlog_len)
 			shortlog_len = DEFAULT_MERGE_LOG_LEN;
 		return 0;
+	} else if (!strcmp(k, "merge.ff")) {
+		int boolval = git_config_maybe_bool(k, v);
+		if (0 <= boolval) {
+			allow_fast_forward = boolval;
+		} else if (v && !strcmp(v, "only")) {
+			allow_fast_forward = 1;
+			fast_forward_only = 1;
+		} /* do not barf on values from future versions of git */
+		return 0;
 	} else if (!strcmp(k, "merge.defaulttoupstream")) {
 		default_to_upstream = git_config_bool(k, v);
 		return 0;

t/t7600-merge.sh

@@ -225,12 +225,28 @@ test_expect_success 'merge c1 with c2 and c3' '
 
 test_debug 'git log --graph --decorate --oneline --all'
 
-test_expect_success 'failing merges with --ff-only' '
+test_expect_success 'merges with --ff-only' '
 	git reset --hard c1 &&
 	test_tick &&
 	test_must_fail git merge --ff-only c2 &&
 	test_must_fail git merge --ff-only c3 &&
-	test_must_fail git merge --ff-only c2 c3
+	test_must_fail git merge --ff-only c2 c3 &&
+	git reset --hard c0 &&
+	git merge c3 &&
+	verify_head $c3
+'
+
+test_expect_success 'merges with merge.ff=only' '
+	git reset --hard c1 &&
+	test_tick &&
+	test_when_finished "git config --unset merge.ff" &&
+	git config merge.ff only &&
+	test_must_fail git merge c2 &&
+	test_must_fail git merge c3 &&
+	test_must_fail git merge c2 c3 &&
+	git reset --hard c0 &&
+	git merge c3 &&
+	verify_head $c3
 '
 
 test_expect_success 'merge c0 with c1 (no-commit)' '
@@ -448,6 +464,29 @@ test_expect_success 'merge c0 with c1 (no-ff)' '
 
 test_debug 'git log --graph --decorate --oneline --all'
 
+test_expect_success 'merge c0 with c1 (merge.ff=false)' '
+	git reset --hard c0 &&
+	git config merge.ff false &&
+	test_tick &&
+	git merge c1 &&
+	git config --remove-section merge &&
+	verify_merge file result.1 &&
+	verify_parents $c0 $c1
+'
+test_debug 'git log --graph --decorate --oneline --all'
+
+test_expect_success 'combine branch.master.mergeoptions with merge.ff' '
+	git reset --hard c0 &&
+	git config branch.master.mergeoptions --ff
+	git config merge.ff false
+	test_tick &&
+	git merge c1 &&
+	git config --remove-section "branch.master" &&
+	git config --remove-section "merge" &&
+	verify_merge file result.1 &&
+	verify_parents "$c0"
+'
+
 test_expect_success 'combining --squash and --no-ff is refused' '
 	test_must_fail git merge --squash --no-ff c1 &&
 	test_must_fail git merge --no-ff --squash c1