Many codepaths forget to check return value from git_config_set();
the function is made to die() to make sure we do not proceed when
setting a configuration variable failed.

* ps/config-error:
  config: rename git_config_set_or_die to git_config_set
  config: rename git_config_set to git_config_set_gently
  compat: die when unable to set core.precomposeunicode
  sequencer: die on config error when saving replay opts
  init-db: die on config errors when initializing empty repo
  clone: die on config error in cmd_clone
  remote: die on config error when manipulating remotes
  remote: die on config error when setting/adding branches
  remote: die on config error when setting URL
  submodule--helper: die on config error when cloning module
  submodule: die on config error when linking modules
  branch: die on config error when editing branch description
  branch: die on config error when unsetting upstream
  branch: report errors in tracking branch setup
  config: introduce set_or_die wrappers