[PATCH v2 2/5] t/lib-commit-graph.sh: avoid directory change in `graph_git_behavior()`

The `graph_git_behavior()` helper asserts that a number of common Git
operations (such as `git log --oneline`, `git log --topo-order`, etc.)
produce identical output regardless of whether or not a commit-graph is
in use.

This helper takes as its second argument the location (relative to the
`$TRASH_DIRECTORY`) of the Git repostiory under test. In order to run
each of its commands within that repository, it first changes into that
directory, without the use of a sub-shell.

This pollutes future tests which expect to be run in the top-level
`$TRASH_DIRECTORY` as usual. We could wrap `graph_git_behavior()` in a
sub-shell, like:

    graph_git_behavior() {
      # ...
      (
        cd "$TRASH_DIRECTORY/$DIR" &&
        graph_git_two_modesl
      )
    }

, but since we're invoking git directly, we can pass along a "-C $DIR"
when "$DIR" is non-empty.

Note, however, that until the remaining callers are cleaned up to avoid
changing working directories outside of a sub-shell, that we need to
ensure that we are operating in the top-level $TRASH_DIRECTORY. The
inner-subshell will go away in a future commit once it is no longer
necessary.

Signed-off-by: Taylor Blau <redacted>
---
 t/lib-commit-graph.sh | 21 +++++++++++++++------
 1 file changed, 15 insertions(+), 6 deletions(-)

diff --git a/t/lib-commit-graph.sh b/t/lib-commit-graph.sh
index 4d3e7f0623e..c8bd76a7777 100755
--- a/t/lib-commit-graph.sh
+++ b/t/lib-commit-graph.sh

@@ -14,18 +14,27 @@ graph_git_two_modes() {
 	test_cmp expect output
 }
 
+# graph_git_behavior <name> <directory> <branch> <compare>
+#
+# Ensures that a handful of traversal operations produce the same
+# results with and without the commit-graph in use.
+#
+# NOTE: it is a bug to call this function with <directory> containing
+# any characters in $IFS.
 graph_git_behavior() {
 	MSG=$1
 	DIR=$2
 	BRANCH=$3
 	COMPARE=$4
 	test_expect_success "check normal git operations: $MSG" '
-		cd "$TRASH_DIRECTORY/$DIR" &&
-		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 $COMPARE"
+		(
+			cd "$TRASH_DIRECTORY" &&
+			graph_git_two_modes "${DIR:+-C $DIR} log --oneline $BRANCH" &&
+			graph_git_two_modes "${DIR:+-C $DIR} log --topo-order $BRANCH" &&
+			graph_git_two_modes "${DIR:+-C $DIR} log --graph $COMPARE..$BRANCH" &&
+			graph_git_two_modes "${DIR:+-C $DIR} branch -vv" &&
+			graph_git_two_modes "${DIR:+-C $DIR} merge-base -a $BRANCH $COMPARE"
+		)
 	'
 }
 

-- 
2.41.0.399.g887006eab46