favor git(1) rather than libgit2 for ExtSearch
While both git and libgit2 take around 16 minutes to load 100K
alternates there's already a proposed patch to make git faster:
<https://lore.kernel.org/git/
20210624005806.12079-1-e@80x24.org/>
It's also easier to patch and install git locally since the
git.git build system defaults to prefix=$HOME and dealing with
dynamic linking with libgit2 is more difficult for end users
relying on Inline::C.
libgit2 remains in use for the non-ALL.git case, but maybe it's
not necessary (libgit2 is significantly slower than git in
Debian 10 due to SHA-1 collision checking).