Refactor list of of repo-local env vars

Refactor list of of repo-local env vars

Move the list of GIT_* environment variables that are local to a
repository into a static list in environment.c, as it is also
useful elsewhere. Also add the missing GIT_CONFIG variable to the
list.

Make it easy to use the list both by NULL-termination and by size;
the latter (excluding the terminating NULL) is stored in the
local_repo_env_size define.

Signed-off-by: Giuseppe Bilotta <giuseppe.bilotta@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>