scalar-diagnose: move 'get_disk_info()' to 'compat/'

scalar-diagnose: move 'get_disk_info()' to 'compat/'

Move 'get_disk_info()' function into 'compat/'. Although Scalar-specific
code is generally not part of the main Git tree, 'get_disk_info()' will be
used in subsequent patches by additional callers beyond 'scalar diagnose'.
This patch prepares for that change, at which point this platform-specific
code should be part of 'compat/' as a matter of convention.

The function is copied *mostly* verbatim, with two exceptions:

* '#ifdef WIN32' is replaced with '#ifdef GIT_WINDOWS_NATIVE' to allow
  'statvfs' to be used with Cygwin.
* the 'struct strbuf buf' and 'int res' (as well as their corresponding
  cleanup & return) are moved outside of the '#ifdef' block.

Signed-off-by: Victoria Dye <vdye@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>