mingw: ensure that core.longPaths is handled *always*

A ton of Git commands simply do not read (or at least parse) the core.*
settings. This is not good, as Git for Windows relies on the
core.longPaths setting to be read quite early on.

So let's just make sure that all commands read the config and give
platform_core_config() a chance.

This patch teaches tons of Git commands to respect the config setting
`core.longPaths = true`, including `pack-refs`, thereby fixing
#1218

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>