Windows: boost startup by avoiding a static dependency on shell32.dll

Windows: boost startup by avoiding a static dependency on shell32.dll

This DLL is only needed to invoke the browser in a "git help" call. By
looking up the only function that we need at runtime, we can avoid the
startup costs of this DLL.

DLL usage can be profiled with Microsoft's Dependency Walker. For example,
a call to "git diff-files" loaded

before:  19 DLLs
after:    9 DLLs

As a result, the runtime of 'make -j2 test' went down from 16:00min
to 12:40min on one of my boxes.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>