Merge branch 'jk/do-not-printf-NULL'

"git config" had a codepath that tried to pass a NULL to printf("%s"), which nobody seems to have noticed. * jk/do-not-printf-NULL: git_config_set_multivar_in_file: handle "unset" errors git_config_set_multivar_in_file: all non-zero returns are errors config: lower-case first word of error strings