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.