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.