write_section() mistakenly treated is input as the whole variable name
instead of simply the section (and possibly subsection) and would
confuse "section.subsection" as a section plus variable name and
produce a wrong section header.
Fix this and include a test for writing "section.subsection.var" and
reading it from the 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.
