Introduce git__reallocarray that checks the product of the number
of elements and element size for overflow before allocation.  Also
introduce git__mallocarray that behaves like calloc, but without the
`c`.  (It does not zero memory, for those truly worried about every
cycle.)

Introduce some helper macros to test integer overflow from arithmetic
and set error message appropriately.

Clean up some sorting function stuff including fixing qsort_r
on MinGW, common function pointer type for comparison, and basic
insertion sort implementation (which we, regrettably, fall back
on for MinGW).

git__bsearch and git__tsort did not pass a payload through to the
comparison function.  This makes it impossible to implement sorted
lists where the sort order depends on external data (e.g. building
a secondary sort order for the entries in a tree).  This commit
adds git__bsearch_r and git__tsort_r versions that pass a third
parameter to the cmp function of a user payload.

This fixes all the warnings on win64 except those in deps, which
come from the regex code.

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

Ensure that all memory related functions (malloc, calloc, strdup, free,
etc) are using their respective `git__` wrappers.

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.

1. The license header is technically not valid if it doesn't have a
copyright signature.

2. The COPYING file has been updated with the different licenses used in
the project.

3. The full GPLv2 header in each file annoys me.