Fix warnings and valgrind issues
This fixes some various warnings that showed up in Travis and
a couple uses of uninitialized memory and one memory leak.
Add config level support in the config API
Added `struct git_config_entry`: a git_config_entry contains the key, the value, and the config file level from which a config element was found.
Added `git_config_open_level`: build a single-level focused config object from a multi-level one.
We are now storing `git_config_entry`s in the khash of the config_file