vcs-svn: set up channel to read fast-import cat-blob response

vcs-svn: set up channel to read fast-import cat-blob response

Set up some plumbing: teach the svndump lib to pass a file descriptor
number to the fast_export lib, representing where cat-blob/ls
responses can be read from, and add a get_response_line helper
function to the fast_export lib to read a line from that file.

Unfortunately this means that svn-fe needs file descriptor 3 to be
redirected from somewhere (preferrably the cat-blob stream of a
fast-import backend); otherwise it will fail:

	$ svndump <path> | svn-fe
	fatal: cannot read from file descriptor 3: Bad file descriptor

For the moment, "svn-fe 3</dev/null" works as a workaround but it
will not work for very long.  A fast-import backend that can retrieve
old commits is needed in order to be able to fulfill svn
"Node-copyfrom-rev" requests that refer to revs from a previous run.

[jn: with new change description]

Based-on-patch-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: David Barr <david.barr@cordelta.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>