guess default branch from HEAD

guess default branch from HEAD

This is a saner alternative than hardcoding the default branch to be
"master". The add_repo() function will now check for a symbolic ref in
repo_path/HEAD. If there is a suitable one, overwrite repo->defbranch
with it. Note that you'll need to strip the newline from the file (->
len-17).

If HEAD is a symbolic link pointing directly to a branch below
refs/heads/, do a readlink() instead to find the ref name.

Signed-off-by: Julius Plenz <plenz@cis.fu-berlin.de>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>