shrink git-shell by avoiding redundant dependencies

shrink git-shell by avoiding redundant dependencies

A lot of modules that have nothing to do with git-shell functionality
were linked in, bloating git-shell more than 8 times.

This patch cuts off redundant dependencies by:
1. providing stubs for three functions that make no sense for git-shell;
2. moving quote_path_fully from environment.c to quote.c to make the
   later self sufficient;
3. moving make_absolute_path into a new separate file.

The following numbers have been received with the default optimization
settings on master using GCC 4.1.2:

Before:
   text    data     bss     dec     hex filename
 143915    1348   93168  238431   3a35f git-shell

After:
   text    data     bss     dec     hex filename
  17670     788    8232   26690    6842 git-shell

Signed-off-by: Junio C Hamano <gitster@pobox.com>