Merge branch 'ks/config-file-stdin'

Merge branch 'ks/config-file-stdin'

"git config" learned to read from the standard input when "-" is
given as the value to its "--file" parameter (attempting an
operation to update the configuration in the standard input of
course is rejected).

* ks/config-file-stdin:
  config: teach "git config --file -" to read from the standard input
  config: change git_config_with_options() interface
  builtin/config.c: rename check_blob_write() -> check_write()
  config: disallow relative include paths from blobs