Merge branch 'ta/config-set-1'

Merge branch 'ta/config-set-1'

Use the new caching config-set API in git_config() calls.

* ta/config-set-1:
  add tests for `git_config_get_string_const()`
  add a test for semantic errors in config files
  rewrite git_config() to use the config-set API
  config: add `git_die_config()` to the config-set API
  change `git_config()` return value to void
  add line number and file name info to `config_set`
  config.c: fix accuracy of line number in errors
  config.c: mark error and warnings strings for translation