`lpExitCode` is a pointer to a long. A long is 32 bits wide on Windows.

It means that on Windows 64bits, `GetExitCodeThread()` doesn't set/clear the high-order bytes of the 64 bits memory space pointed at by `value_ptr`.

The indexer was creating a packfile object separately from the
code in pack.c which was a problem since I put a call to
git_mutex_init into just pack.c.  This commit updates the pack
function for creating a new pack object (i.e. git_packfile_check())
so that it can be used in both places and then makes indexer.c
use the shared initialization routine.

There are also a few minor formatting and warning message fixes.

Incorporate feedback for incr. improvements to pack-objects

This converts blob.c, fileops.c, and all of the win32 files.
Also, various minor cleanups throughout the code.  Plus, in
testing the win32 build, I cleaned up a bunch (although not
all) of the warnings with the 64-bit build.

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.

Signed-off-by: schu <schu-github@schulog.org>

There were quite a few places were spaces were being used instead of
tabs. Try to catch them all. This should hopefully not break anything.
Except for `git blame`. Oh well.