Handle symbolic heads

Handle symbolic heads

RCS files can contain symbols that don't point to an actual commit;
checking out these symbols retrieves the highest revision with matching
major numbers. Fo example, with symbol SYM:3 and revisions 2.1, 3.1,
3.2, 4.1, checking out SYM results in 3.2 being checked out.

This feature translates pretty straightforwardly to git heads. These
symbols are detected by their missing date and are deleted after
assigning their name as branch to the appropriate revision.