Drop use of $(...) for popen() and system() calls

Drop use of $(...) for popen() and system() calls

Some shells don't support it, like the jsh that is used as /bin/sh on
some Solaris boxes, which results in the following error:

	sh: syntax error at line 1: `(' unexpected

Fix the only user (reading remote information via git-ls-remote) to use
a much simpler shell syntax.

Reported by SungHyun Nam.