[log] Fail safer if no refs were found at some head commit

Returning a NullVirtualFile won't help, because it just fails later
in the DataGetter in attempt to load commit details from the VCS.

Returning the first root is better: it will suit in most cases
(one-repo case), although it can easily fail in multi-repo case.
[log] Don't report the error too often
[git] Don't log error in case of empty config file - it is allowed
[git] IDEA-124052 Support remote refs w/o a correspondent remote + test
Cleanup (unneeded interface)
IDEA-123956 Anonymous class formatting moves all args to the next line [CR-IC-5212]
remove alarm. Make SE faster