delta-islands: free island-related data after use

delta-islands: free island-related data after use

On my use case involving 771 islands of Linux on kernel.org,
this reduces memory usage by around 25MB.  The bulk of that
comes from free_remote_islands, since free_config_regexes only
saves around 40k.

This memory is saved early in the memory-intensive pack process,
making it available for the remainder of the long process.

Signed-off-by: Eric Wong <e@80x24.org>
Co-authored-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>