Makefile: bump Git version to v2.44.0.gl2

We have recently tried to roll out Git v2.44.0 to production systems but discovered that it breaks object reuse in repositories with a multi-pack index. This issue led to user-visible errors as well as object reuse being disabled completely when serving fetches.

We have fixed the issue upstream and backported the fix to our custom Git v2.44.0.gl2. Upgrade to that version to unblock the rollout.

Closes git#298 (closed).

Part of #5906 (closed).