config: read config from a repository object

config: read config from a repository object

Teach the config machinery to read config information from a repository
object.  This involves storing a 'struct config_set' inside the
repository object and adding a number of functions (repo_config*) to be
able to query a repository's config.

The current config API enables lazy-loading of the config.  This means
that when 'git_config_get_int()' is called, if the_config_set hasn't
been populated yet, then it will be populated and properly initialized by
reading the necessary config files (system wide .gitconfig, user's home
.gitconfig, and the repository's config).  To maintain this paradigm,
the new API to read from a repository object's config will also perform
this lazy-initialization.

Since both APIs (git_config_get* and repo_config_get*) have the same
semantics we can migrate the default config to be stored within
'the_repository' and just have the 'git_config_get*' family of functions
redirect to the 'repo_config_get*' functions.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>