git is isolated using the following environment, which are set globally
for the duration of that the new pytest fixture "tmp_git" is used:

GIT_CONFIG_GLOBAL
GIT_CONFIG_NOSYSTEM
HOME
GIT_AUTHOR_EMAIL
GIT_AUTHOR_NAME
GIT_AUTHOR_DATE
GIT_COMMITTER_EMAIL
GIT_COMMITTER_NAME
GIT_COMMITTER_DATE

GIT_DIR and GIT_WORK_TREE could be set so that git can be called using
any method, but instead a git() function is added that uses git's -C
command-line option.

These were taken from one of git's test scripts:
https://github.com/git/git/blob/cefe983a320c03d7843ac78e73bd513a27806845/t/test-lib.sh#L454-L461

There are probably other ways git can be isolated.

The repository is initialized with an empty commit, but this isn't
strictly necessary, it just makes some of the possible tests require
less setup.

The new tmp_project fixture copies the sample module from
./test/samples/module1_toml to the project and commits the files.

A test is added for pypa#345 as an example of how this can be used.

A pytest marker is added so that tests with either "needgit" or
"needsgit" in the name are skipped if python can't find an executable
named "git".

The tox configuration is changed and another pytest marker is added so
that those tests can be run by themselves:

tox -- -m needgit

or skipped:

tox -- -m "not needgit"

Type hints were added to help with development, but aren't necessary to
keep.