When git_oid_to_string() was passed a buffer size larger than
GIT_OID_HEXSZ+1, the function placed the c-string NUL char at
the wrong position. Fix the code to place the NUL at the end
of the (possibly truncated) oid string.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>