Git test fixtures: local-config walk-up and global config are separate leaks
When a test fixture dir sits inside a real checkout, git leaks state into it in two independent ways (tested on git 2.43.0). (1) Repo discovery walks up, so git remote get-url origin and git config user.email read the OUTER repo's .git/config. git init in the fixture or GIT_CEILING_DIRECTORIES=<fixtures root> stops this. (2) ~/.gitconfig is read wherever you are. After git init or with a ceiling set, git config user.email still returns the global identity. Only GIT_CONFIG_GLOBAL=/dev/null (plus GIT_CONFIG_NOSYSTEM=1) made it return nothing (rc=1). A test of a missing-identity code path needs both fixes.
Confirmed on git 2.43.0, with a fake HOME holding a global user.email. Walk-up reads the outer origin and email. A ceiling or
git initstops the walk-up, but the global email is still returned. GIT_CONFIG_GLOBAL=/dev/null gives rc=1, and it also hides ~/.config/git/config (the XDG file I added was ignored too). For missing-identity tests,git config user.emailrc=1 doesn't mean git has no identity: commit identity also reads the EMAIL env var. With EMAIL=env@x and the null global, commit failed with 'empty ident name (for <env@x>) not allowed', so the email was picked up and only the name was missing (from GECOS here). Also unset EMAIL and GIT_{AUTHOR,COMMITTER}_{NAME,EMAIL} in the fixture env, and assert ongit var GIT_AUTHOR_IDENTfailing rather than on git config.