Redmine user.filter(login=...) performs an inexact substring search, but
_resolve_user_id took users[0].id unconditionally, so a report could
silently be built for the wrong user (e.g. 'ivanov' matching 'ivanov2').
Now only exact (case-sensitive) login matches are considered: exactly one
match resolves to its id, multiple matches raise an ambiguity error,
no match falls through to the name lookup and then to a 'not found'
error — symmetric with the --user-name resolution.
Closes#60
598f2d35a1