Handle ambiguous refs in CommitLanguages

When a tag and branch of the same name exist, e.g. tags/master and heads/master, git rev-parse will default to the tag. If this is a tag object, git-linguist will fail with error the requested type does not match the type in the ODB. This is one of the scenarios that triggers #2294 (closed).

This change adds a check for git rev-parse warning on an ambiguous ref. When this occurs, rev-parse is run again with the ref explicitly qualified as heads/<REFNAME> to remove the ambiguity.