Bump minimum version of Git to 2.40

Once Git 2.40 is rolled out (#4982 (closed)), we can bump the minimum version of Git to 2.40. Features that this allows us to use unconditionally:

• git-patch-id(1) was fixed to always respect binaries, which is important to properly detect changes in MRs.
• git-multi-pack-index(1) has learned to delete redundant bitmaps, which is a slight annoyance for us in tests. It's also not great though that these can stay around in production systems.
• git-cat-file(1)'s --batch-check mode has learned to honor mailmaps. This unblocks #4364 (closed).
• git-merge-tree(1) has gained some additions that may make it more useful for us, like --stdin or --merge-base.
• git-check-attr(1) has learned to take a treeish to read the gitattributes file.