Default core.packdGitWindowSize to 1 MiB if NO_MMAP.

If the compiler has asked us to disable use of mmap() on their
platform then we are forced to use git_mmap and its emulation via
pread.  In this case large (e.g. 32 MiB) windows for pack access
are simply too big as a command will wind up reading a lot more
data than it will ever need, significantly reducing response time.

To prevent a high latency when NO_MMAP has been selected we now
use a default of 1 MiB for core.packedGitWindowSize.  Credit goes
to Linus and Junio for recommending this more reasonable setting.

[jc: upcased the name of the symbolic constant, and made another
 hardcoded constant into a symbolic constant while at it. ]

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

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