This significantly reduces I/O when building profiles, especially with
lots of package-specific sub-directories (such as 'share/emacs/24.3',
'texmf', etc.)
* guix/build/union.scm (union-build)[file-tree](others-have-it?): New
procedure. Use it in the 'enter?' parameter of 'file-system-fold';
change 'skip' parameter accordingly.
* tests/union.scm ("union-build"): Ensure that 'include' is a symlink
and 'bin' is a directory.
* guix/build/union.scm (union-build): Prepend "." to the result of
`union-tree', to match the expectations of `delete-duplicate-leaves'.
Don't do mkdir when SUBDIR is ".".
* tests/union.scm ("union-build"): Keep duplicates in %BOOTSTRAP-INPUTS.
* guix/build/union.scm (delete-duplicate-leaves): New procedure.
(union-build)[leaf=?, resolve-collision]: New procedures.
Use `delete-duplicate-leaves' on the result of `tree-union'.
* tests/union.scm ("delete-duplicate-leaves, default",
"delete-duplicate-leaves, file names"): New tests.
* tests/derivations.scm (%bash): New variable.
Replace occurrences of "/bin/sh" by %BASH.
* tests/union.scm ("union-build"): Delete duplicates from
%BOOTSTRAP-INPUTS.
* tests/builders.scm (network-reachable?): New variable.
("url-fetch", "gnu-build"): Skip unless NETWORK-REACHABLE?.
* tests/derivations.scm (%coreutils): Check for network access.
("build-expression->derivation with one input"): Skip when %COREUTILS
is #f.
* tests/guix-package.sh: Skip installation of GNU Make when the network
is unreachable.
* tests/packages.scm ("GNU Make, bootstrap"): Likewise.
* tests/union.scm ("union-build"): Likewise.
