It is not really helpful when a `git fetch` fails with the message:

	fatal: mmap failed: No error

In the particular instance encountered by a colleague of yours truly,
the Win32 error code was ERROR_COMMITMENT_LIMIT which means that the
page file is not big enough.

Let's make the message

	fatal: mmap failed: File too large

instead, which is only marginally better, but which can be associated
with the appropriate work-around: setting `core.packedGitWindowSize` to
a relatively small value.

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

Often we are mmap()ing read-only. In those cases, it is wasteful to map in
copy-on-write mode. Even worse: it can cause errors where we run out of
space in the page file.

So let's be extra careful to map files in read-only mode whenever
possible.

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

On Windows, we have to emulate the fstat() call to fill out information
that takes extra effort to obtain, such as the file permissions/type.

If all we want is the file size, we can use the much cheaper
GetFileSizeEx() function (available since Windows XP).

Suggested by Philip Kelley.

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