Explanation for dropping write-back in mmap

Hi Junio,

In the commit f48000fcbe100, you've forced the caller to use
MAP_PRIVATE in the alternative mmap implementation dropping write-back
support. Could you kindly explain the rationale for this? David's SVN
exporter uses mmap with MAP_SHARED, and we figured that the merge
immediately breaks this functionality.

Noticed-by: Jonathan Nieder [off-list ref]

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commit f48000fcbe1009c18f1cc46e56cde2cb632071fa
Author: Junio C Hamano [off-list ref]
Date:   Sat Oct 8 15:54:36 2005 -0700

    Yank writing-back support from gitfakemmap.

    We do not write through our use of mmap(), so make sure callers pass
    MAP_PRIVATE and remove support for writing changes back.

    Signed-off-by: Junio C Hamano [off-list ref]
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Thanks.

-- Ram