commit-graph tests: fix another graph_git_two_modes() helper

In 135a712375 (commit-graph: add --split option to builtin,
2019-06-18) this function was copy/pasted to the split commit-graph
tests, as in the preceding commit we need to fix this to use
&&-chaining, so it won't be hiding errors.

Unlike its sister function in "t5318-commit-graph.sh", which we got
lucky with, this one was hiding a real test failure. A tests added in
c523035cbd (commit-graph: allow cross-alternate chains, 2019-06-18)
has never worked as intended. Unlike most other graph_git_behavior
uses in this file it clones the repository into a sub-directory, so
we'll need to refer to "commits/6" as "origin/commits/6".

It's not easy to simply move the "graph_git_behavior" to the test
above it, since it itself spawns a "test_expect_success". Let's
instead add support to "graph_git_behavior()" and
"graph_git_two_modes()" to pass a "-C" argument to git.

We also need to add a "test -d fork" here, because otherwise we'll
fail on e.g.:

    GIT_SKIP_TESTS=t5324.13 ./t5324-split-commit-graph.sh

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5324-split-commit-graph.sh

@@ -55,8 +55,8 @@ test_expect_success 'create commits and write commit-graph' '
 '
 
 graph_git_two_modes() {
-	git -c core.commitGraph=true $1 >output
-	git -c core.commitGraph=false $1 >expect
+	git ${2:+ -C "$2"} -c core.commitGraph=true $1 >output &&
+	git ${2:+ -C "$2"} -c core.commitGraph=false $1 >expect &&
 	test_cmp expect output
 }
 
@@ -64,12 +64,13 @@ graph_git_behavior() {
 	MSG=$1
 	BRANCH=$2
 	COMPARE=$3
+	DIR=$4
 	test_expect_success "check normal git operations: $MSG" '
-		graph_git_two_modes "log --oneline $BRANCH" &&
-		graph_git_two_modes "log --topo-order $BRANCH" &&
-		graph_git_two_modes "log --graph $COMPARE..$BRANCH" &&
-		graph_git_two_modes "branch -vv" &&
-		graph_git_two_modes "merge-base -a $BRANCH $COMPARE"
+		graph_git_two_modes "log --oneline $BRANCH" "$DIR" &&
+		graph_git_two_modes "log --topo-order $BRANCH" "$DIR" &&
+		graph_git_two_modes "log --graph $COMPARE..$BRANCH" "$DIR" &&
+		graph_git_two_modes "branch -vv" "$DIR" &&
+		graph_git_two_modes "merge-base -a $BRANCH $COMPARE" "$DIR"
 	'
 }
 
@@ -187,7 +188,10 @@ test_expect_success 'create fork and chain across alternate' '
 	)
 '
 
-graph_git_behavior 'alternate: commit 13 vs 6' commits/13 commits/6
+if test -d fork
+then
+	graph_git_behavior 'alternate: commit 13 vs 6' commits/13 origin/commits/6 "fork"
+fi
 
 test_expect_success 'test merge stragety constants' '
 	git clone . merge-2 &&