allocations: test for overflow of requested size

Fixups for rebase

Switch to a standard branch-switching pattern of a `SAFE` checkout,
then updating `HEAD`.

Don't require the branch to rebase, if given `NULL`, simply look up
`HEAD`.

Users may want to try to pay attention to the `exec` field on all
rebase operations.

win32: limit the mode to `_wopen`/`_waccess`

gcc and clang support __builtin_add_overflow, use it whenever
possible, falling back to our naive routines.

Make our overflow checking look more like gcc and clang's, so that
we can substitute it out with the compiler instrinsics on platforms
that support it.  This means dropping the ability to pass `NULL` as
an out parameter.

As a result, the macros also get updated to reflect this as well.

Win32 generally ignores Unix-like mode bits that don't make any
sense on the platform (eg `0644` makes no sense to Windows).  But
WINE complains loudly when presented with POSIXy bits.  Remove them.

(Thanks @phkelley)

Reinit `reader` pointer after reading included config file

Fix build warning on Android

https://github.com/attractivechaos/klib/pull/42/files introduces
`kreallocarray`.  Hook that up as our `git__reallocarray`.

Add some helper functions to check for overflow in a type-specific
manner.

Ensure that the given length to `p_read` is of ssize_t and ensure
that callers test the return as if it were an `ssize_t`.

Use `size_t` to hold the size of arrays to ease overflow checking,
lest we check for overflow of a `size_t` then promptly truncate
by packing the length into a smaller type.

Have the ALLOC_OVERFLOW testing macros also simply set_oom in the
case where a computation would overflow, so that callers don't
need to.

Introduce `git_buf_grow_by` to incrementally increase the size of a
`git_buf`, performing an overflow calculation on the growth.

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.

Always do a time_t cast on st_mtime. st_mtime on Android is not
the type of time_t but has the same meaning which is the number
of seconds past epoch.

Check rebase options version on public entry points

Fixes #2869. If included file includes more files, it may reallocate
cfg_file->readers, hence invalidate not only `r` pointer, but `result`
pointer as well.

It fails at least on my computer, though it may depend on some unpredictable
factors (say, will realloc() extend the memory segment in place, or it will
allocate new memory).

Checkout performance

Remove stale reflog namespace directory before branch creation

Ensure we can make a repo at the root of the filesystem

Add libssh2's library and include directories.

Large merge perf test.

Update some third-party copyrights

Fix doc comment formatting