git.py: add support for passing in a stdin input stream
git.py: add support for passing in a stdin input stream

execute/method_missing now support:

	fh = open('filename', 'r')
	print git.foo( istream=fh )
	fh.close()

The name "istream" was chosen so that it would not conflict
with any of the builtin git flags.  "stdin", for instance,
is used by git.

Signed-off-by: David Aguilar <9de348c050f7cd1ca590883733c4e531ce610bf4@gmail.com>

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