builtins: Makefile clean-up

builtins: Makefile clean-up

This cleans up the build procedure for built-in commands by:

 - generating mostly redundant definition of BUILT_INS from
   BUILTIN_OBJS in the Makefile,
 - renaming a few files to make the above possible, and
 - sorting the built-in command table in git.c.

It might be a good idea to binary search (or perfect hash) the built-in
command table, but that can be done later when somebody feels like.

Signed-off-by: Junio C Hamano <redacted>