merge: let custom strategies intervene in trivial merges

As v1.6.1-rc1~294^2 (2008-08-23) explains, custom merge strategies
do not even kick in when the merge is truly trivial.  But they
should, since otherwise a custom “--strategy=theirs” is not useful.

Perhaps custom strategies should not allow fast-forward either.  This
patch does not make that change, since it is less important (because
it is always possible to explicitly use --no-ff).

Reported-by: Yaroslav Halchenko <debian@onerussian.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/merge.c

@@ -131,6 +131,7 @@ static struct strategy *get_strategy(const char *name)
 
 	ret = xcalloc(1, sizeof(struct strategy));
 	ret->name = xstrdup(name);
+	ret->attr = NO_TRIVIAL;
 	return ret;
 }
 

t/t7606-merge-custom.sh

@@ -4,11 +4,13 @@ test_description="git merge
 
 Testing a custom strategy.
 
-*   (HEAD, master) Merge commit 'c2'
+*   (HEAD, master) Merge commit 'c3'
 |\
-| * (tag: c2) c2
+| * (tag: c3) c3
 * | (tag: c1) c1
 |/
+| * tag: c2) c2
+|/
 * (tag: c0) c0
 "
 
@@ -31,7 +33,9 @@ test_expect_success 'setup' '
 	git reset --keep c0 &&
 	echo c1c1 >c1.c &&
 	git add c1.c &&
-	test_commit c2 c2.c
+	test_commit c2 c2.c &&
+	git reset --keep c0 &&
+	test_commit c3 c3.c
 '
 
 test_expect_success 'merge c2 with a custom strategy' '
@@ -60,4 +64,30 @@ test_expect_success 'merge c2 with a custom strategy' '
 	test -f c2.c
 '
 
+test_expect_success 'trivial merge with custom strategy' '
+	git reset --hard c1 &&
+
+	git rev-parse c1 >head.old &&
+	git rev-parse c3 >second-parent.expected &&
+	git rev-parse c3^{tree} >tree.expected &&
+	git merge -s theirs c3 &&
+
+	git rev-parse HEAD >head &&
+	git rev-parse HEAD^1 >first-parent &&
+	git rev-parse HEAD^2 >second-parent &&
+	git rev-parse HEAD^{tree} >tree &&
+	git update-index --refresh &&
+	git diff --exit-code &&
+	git diff --exit-code c3 HEAD &&
+	git diff --exit-code c3 &&
+
+	! test_cmp head.old head &&
+	test_cmp head.old first-parent &&
+	test_cmp second-parent.expected second-parent &&
+	test_cmp tree.expected tree &&
+	test -f c0.c &&
+	! test -e c1.c &&
+	test -f c3.c
+'
+
 test_done