Give the rebase integration tests a repo-local git identity

The new integration tests drive the production rebase helpers, which shell out
to git without the identity environment variables gitExec sets, so creating a
commit during a rebase failed on a runner with no global git identity — passing
locally and on macOS but failing on ubuntu-latest.

Recording the identity in the clone's own config covers every git process
started in it, whatever path reaches git. Verified by running the whole suite
with GIT_CONFIG_GLOBAL and GIT_CONFIG_SYSTEM pointed at /dev/null.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: f5959297-80fd-4732-aeae-aa9a4b6a7755

Author: skarim

internal/git/rebase_start_test.go

@@ -14,6 +14,13 @@ func stackedRepo(t *testing.T) string {
 	t.Helper()
 	_, clone := setupBareAndClone(t)
 
+	// These tests drive the production rebase helpers, which shell out to git
+	// without the identity env vars gitExec sets. Record it in the repo config
+	// so any git process started in this clone can create commits, including on
+	// machines with no global identity configured.
+	gitExec(t, clone, "config", "user.name", "Test")
+	gitExec(t, clone, "config", "user.email", "test@test.com")
+
 	writeFile(t, clone, "trunk.txt", "trunk v1")
 	gitExec(t, clone, "add", ".")
 	gitExec(t, clone, "commit", "-m", "trunk commit")