builtin/gc.c: correct RAM calculation when using sysinfo

builtin/gc.c: correct RAM calculation when using sysinfo

The man page for sysinfo(2) on Linux states that (from v2.3.48) the
sizes of the memory and swap fields, of the returned structure, are
given as multiples of 'mem_unit' bytes. In earlier versions (prior to
v2.3.23 on i386 in particular), the 'mem_unit' field was not part of
the structure, and all sizes were measured in bytes. The man page does
not discuss the motivation for this change, but it is possible that the
change was intended for the, relatively rare, 32-bit platform with more
than 4GB of memory.

The total_ram() function makes the assumption that the 'totalram' field
of the 'struct sysinfo' is measured in bytes, or alternatively that the
'mem_unit' field is always equal to one. Having writen a program to call
the sysinfo() function and print the structure fields, it seems that, on
Linux x84_64 and i686 anyway, the 'mem_unit' field is indeed set to one
(note that the 32-bit system had only 2GB ram). However, cygwin also has
an sysinfo() implementation, which gives the following values:

  $ ./sysinfo
  uptime:      21381
  loads:       0, 0, 0
  total ram:   2074637
  free ram:    843237
  shared ram:  0
  buffer ram:  0
  total swap:  327680
  free swap:   306932
  procs:       15
  total high:  0
  free high:   0
  mem_unit:    4096

  total ram: 8497713152
  $

[This laptop has 8GB ram, so a little bit seems to be missing. ;) ]

Modify the total_ram() function to allow for the possibility that the
memory size is not specified in bytes (ie 'mem_unit' is greater than
one).

Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>