[CHORE] Drop go-git support

See https://codeberg.org/forgejo/discussions/issues/164 for the
rationale and discussion of this change.

Everything related to the `go-git` dependency is dropped (Only a single
instance is left in a test file to test for an XSS, it requires crafting
an commit that Git itself refuses to craft). `_gogit` files have
been removed entirely, `go:build: !gogit` is removed, `XXX_nogogit.go` files
either have been renamed or had their code being merged into the
`XXX.go` file.

modules/git/utils_test.go

@@ -13,7 +13,7 @@ import (
 // but not in production code.
 
 func skipIfSHA256NotSupported(t *testing.T) {
-	if isGogit || CheckGitVersionAtLeast("2.42") != nil {
+	if CheckGitVersionAtLeast("2.42") != nil {
 		t.Skip("skipping because installed Git version doesn't support SHA256")
 	}
 }