t0450: fix test for out-of-tree builds
When using Meson, builds are out-of-tree and $GIT_BUILD_DIR gets set to
the path where the build output is landing. To locate the Documentation
sources, test 't0450' was using that path.
Modify test 't0450' to use `$GIT_SOURCE_DIR/Documentation` to find the
documentation sources.
Signed-off-by: Toon Claes <toon@iotcl.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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