MSVC: allow using ExtUtils::MakeMaker

Drop NO_PERL_MAKEMAKER from config.mak.uname for the MSVC platform.

MakeMaker is available on Windows Perl implementations and
installs modules to correct location, unlike NO_PERL_MAKEMAKER Makefile.

Signed-off-by: Marat Radchenko <marat@slonopotamus.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Add support for commit attributes.

Commit objects have extended attributes using '-A' or '--attr' flag to
add them in git-commit command. Also, commit attributes are gotten using
'%A(...)' format in git-log and/or git-show command. Examples:

git commit -m "Commit message." --attr name=value
git log -1 --format="%A(name)" # Shows 'value'.

In addition, documentation and tests have been added.