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 in 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 if manually set g:ivicc_reponame
global variable in configuration file.