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

diff --cc Documentation/technical/api-config.txt
Simple merge

diff --cc cache.h
Simple merge

diff --cc config.c
Simple merge