multipack: Introduce a parser for multi-pack-index files

This change is the first in a series to add support for git's
multi-pack-index. This should speed up large repositories significantly.

Part of: #5399

fuzzers/corpora/midx/0903e378a493c596298074d6bff8de7f9ac25aa7

+7
\ No newline at end of file

fuzzers/corpora/midx/09144a846f90f894049ef8a0ed0cc7ab4588dc6c

+seed
\ No newline at end of file

fuzzers/corpora/midx/0f6c5fc9b6a68835364bbef8937560ee5a481938

+*
\ No newline at end of file

fuzzers/corpora/midx/118735f7786ae6b4c2f6b36314ab1f2cafe9c3c8

+ۊ
\ No newline at end of file

fuzzers/corpora/midx/1c9599ce00978780519272be279f508c402e3268

+
\ No newline at end of file

fuzzers/corpora/midx/341773a439cdecc58f55fb205ac584cd93ffe0f2

+yyyyyyyyyyyyy
\ No newline at end of file

fuzzers/corpora/midx/38ddf3424559f1a6e7687eff8469a358184b833b

+D
\ No newline at end of file