Ask user about repository's name for variating URL

Current g:ivicc_commit_url-s won't work anymore! You have to modify this
URL template: add %reponame and replace existing "%" with "%commit".

Different repositories will vary URL for their commits. There is no
standard way to determine it. Ask user about repository's name once
during session. One can omit that step by manually setting
g:ivicc_reponame in configuration file.