create_symref(): do not assume pathname from git_path() persists long enough

create_symref(): do not assume pathname from git_path() persists long enough

Being lazy to rely on the cycling N buffers mkpath() and friends
return is nice in general, but it makes it too easy to introduce
new bugs that are "mysterious".

Introduction of read_ref() in create_symref() after calling
git_path() to get the git_HEAD value (i.e. the path to create a
new symref at) consumed more than the available buffers and
broke a later call to mkpath() that derives lockpath from it.

Signed-off-by: Junio C Hamano <junkio@cox.net>