Makefile: list standalone program object files in PROGRAM_OBJS

Makefile: list standalone program object files in PROGRAM_OBJS

Because of new commands like git-remote-http, the OBJECTS list
contains fictitious objects such as remote-http.o.  Thus any
out-of-tree rules that require all $(OBJECTS) to be buildable
are broken.  Add a list of real program objects to avoid this
problem.

To avoid duplication of effort, calculate the command list in
the PROGRAMS variable using the expansion of PROGRAM_OBJS.
This calculation occurs at the time $(PROGRAMS) is expanded,
so later additions to PROGRAM_OBJS will be reflected in it,
provided they occur before the build rules begin on line 1489.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>