fix test comment
improve inter-documentation linkage by using L<> instead of C<>
use Git::Repository in the test scripts

some of the code still uses Git.pm:
- all the Git::FastExport and Git::FastExport::Stitch code
- repo_description in t::Utils
add support for Git::Repository objects to repo_description

still support the old Git.pm objects for now
pass Git::Repository objects to repo_description() again
use Test::Git to ensure minimal git version
use Git::Repository in Git::FastExport

and fix all the tests
remove the remaining Git.pm-related code from t::Utils
shorten new_repo()
bring back the possibility to set export_fh directly
allow the path parameter to Git::FastExport->new to be either GIT_DIR or GIT_WORK_TREE
no need to cache the test repositories any more
Merge branch 'git-repository'
documentation improvements
silence all git warnings in tests