Move logic of "mkbranch_versions" and "checkin_versions" to a separate function

After a code review in charleso/git-cc#48.
Move common logic to "empty_file_versions" function

After a code review in charleso/git-cc#48.
Perform versions date filter before calling "checkin_versions"

Store to "actual_versions" filtered versions by date and use
it in "checkin_versions" and "empty_file_versions" functions.
Remove "date" parameter in all functions because "versions"
had already become filtered.