Check whether the backslash at the end of the line is being escaped or
not so as not to consider it a continuation marker when it's e.g. a
Windows-style path.
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
