RFC: Some cleanups/features in CMakeLists.txt

Fix some leaks in tests

Also adds some asserts.

... so we can clean up correctly.

Instead tell MSVC to group the source files by directory.

- Also document the -DSTDCALL even better.

With the src/compat/*.c glob removed, there is no longer a difference to
the default globs we use for the IF( UNIX ) case.

This directory doesn't exist.

Up to now, on windows we don't even bother to look if the user has a zlib
available somwhere.

In almost all larger commercial projects i've participated in, it was not
at all uncommon to have such a dependency somewhere in the source tree and
use it whereever required.
Searching for it, even if it's unlikely to be present, allows for such a
scenario (i.e. by prefilling the CMake-Cache).

don't deref before we've asserted

just sayin'.

Trivial comment fix

I was totally flaunting @ben's 3-space tab advice.

This looked wrong to me.  I *think* this is more appropriate
commentary.

Reset global variable to NULL after free'ing resource

Fix diff constructor names

Based on feedback from the ObjectiveGit folks, these are some
further updates to diff.h areas that are poorly documented.

The diff constructor functions had some confusing names, where the
"old" side of the diff was coming after the "new" side.  This
reverses the order in the function name to make it less confusing.

Specifically...

* git_diff_index_to_tree becomes git_diff_tree_to_index
* git_diff_workdir_to_index becomes git_diff_index_to_workdir
* git_diff_workdir_to_tree becomes git_diff_tree_to_workdir

Introduce git_blob_is_binary()

Cleanup the empty-clone test correctly

Allow clone to handle empty repos

adding .mailmap file

Help clarify who are the top commiters when using 'shortlog -sn'.