Fix compilation when NO_CURL is defined

Fix compilation when NO_CURL is defined

There were a few places which did not cope well without curl.  This
fixes all of them.  We still need to link against the walker.o part
of the library as some parts of transport.o still call into there
even though we don't have HTTP support enabled.

If compiled with NO_CURL=1 we now get the following useful error
message:

  $ git-fetch http://www.example.com/git
  error: git was compiled without libcurl support.
  fatal: Don't know how to fetch from http://www.example.com/git

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>