bundle-uri: unit test "key=value" parsing

bundle-uri: unit test "key=value" parsing

Create a new 'test-tool bundle-uri' test helper. This helper will assist
in testing logic deep in the bundle URI feature.

This change introduces the 'parse-key-values' subcommand, which parses
an input file as a list of lines. These are fed into
bundle_uri_parse_line() to test how we construct a 'struct bundle_list'
from that data. The list is then output to stdout as if the key-value
pairs were a Git config file.

We use an input file instead of stdin because of a future change to
parse in config-file format that works better as an input file.

Co-authored-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>