Actually allow TAG_FIXUP branches in fast-import

Actually allow TAG_FIXUP branches in fast-import

Michael Haggerty <mhagger@alum.mit.edu> noticed while debugging a
Git backend for cvs2svn that fast-import was barfing when he tried
to use "TAG_FIXUP" as a branch name for temporary work needed to
cleanup the tree prior to creating an annotated tag object.

The reason we were rejecting the branch name was check_ref_format()
returns -2 when there are less than 2 '/' characters in the input
name.  TAG_FIXUP has 0 '/' characters, but is technically just as
valid of a ref as HEAD and MERGE_HEAD, so we really should permit it
(and any other similar looking name) during import.

New test cases have been added to make sure we still detect very
wrong branch names (e.g. containing [ or starting with .) and yet
still permit reasonable names (e.g. TAG_FIXUP).

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>