restore: Use bitmap index from backup repo, if present
This way, if backup repository was freshly repacked with bitmap index generation turned on, we can get ~ 30% - 50% speedup for a typical erp5.git pack extraction. "--use-bitmap-index" option was added to git in v2.0, but was only active for to-stdout packs generation. It was enabled for to-file packs generation in git v2.11. Since git v2.0 was released in 2014 - 4 years ago - I'm not adding runtime detection of "--use-bitmap-index" availability. See https://git.kernel.org/pub/scm/git/git.git/commit/?h=645c432d61 for details.
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