We used to require loose references to contain only an OID (possibly
after trimming the string). This is however not enough for letting us
lookup FETCH_HEAD, which can have a lot of content after the initial
OID.

Change the parsing rules so that a loose refernce must e at least 40
bytes long and the 41st (if it's there) must be accepted by
isspace(3). This makes the trim unnecessary, so only do it for
symrefs. This fixes #977.