khash: name the structs that khash declares

khash.h lets you instantiate custom hash types that map between two types. These are defined as a struct, as you might expect, and khash typedef's that to kh_foo_t. But it declares the struct anonymously, which doesn't give a name to the struct type itself; there is no "struct kh_foo". This has two small downsides: - when using khash, we declare "kh_foo_t *the_foo". This is unlike our usual naming style, which is "struct kh_foo *the_foo". - you can't forward-declare a typedef of an unnamed struct type in C. So we might do something like this in a header file: struct kh_foo; struct bar { struct kh_foo *the_foo; }; to avoid having to include the header that defines the real kh_foo. But that doesn't work with the typedef'd name. Without the "struct" keyword, the compiler doesn't know we mean that kh_foo is a type. So let's always give khash structs the name that matches our conventions ("struct kh_foo" to match "kh_foo_t"). We'll keep doing the typedef to retain compatibility with existing callers. Co-authored-by: Jeff King <peff@peff.net> Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Elijah Newren <newren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>