tests: remove 'hghave symlink' from test-import-git.t
The test had a long chain of commits depending on execbits in some of first
commits. The hashes are checked throughout the file, so there was no elegant
way to make it pass both with and without execbits.
We now rollback the execbits-or-not commits and make a stable change instead.
The hash chain is thus updated once but is now a bit more stable. The test
coverage should be unaltered.