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
It turns out that commit 31e9cfc4cbcaf1b38cdd3dbe3282a8f57e5366a5
did not fix the GIT_USUSED behavior on all platforms. This commit
walks through and really cleans things up more thoroughly, getting
rid of the unnecessary stuff.
To remove the use of some GIT_UNUSED, I ended up adding a couple
of new iterators for hashtables that allow you to iterator just
over keys or just over values.
In making this change, I found a bug in the clar tests (where we
were doing *count++ but meant to do (*count)++ to increment the
value). I fixed that but then found the test failing because it
was not really using an empty repo. So, I took some of the code
that I wrote for iterator testing and moved it to clar_helpers.c,
then made use of that to make it easier to open fixtures on a
per test basis even within a single test file.
