Fix debug include printing for multi-source

[mwichmann]

Rather obscure bug - if --debug=includes is selected, the analysis stopped after the first source file of a given target. For the common case of C/C++ projects, the object builders are single-source, so the problem would not be noticed.

Adds a distinct testcase, because the main debug-includes test uses the C compiler, which won't trigger this.

No doc impacts, although the documentation of the option could perhaps be a little more explicit.

Contributor Checklist:

• I have created a new test or updated the unit tests to cover the new/changed functionality.
• I have updated CHANGES.txt and RELEASE.txt (and read the README.rst).
• I have updated the appropriate documentation

[mwichmann]

Just so it's mentioned here, do the printouts make sense? For source file f2.in, we get this display:

+-f2.in
  +-a.inc
  | +-[a.inc]
  | +-b.inc
  |   +-[a.inc]
  |   +-[b.inc]
  +-[b.inc]

Why all the extra noise? a.inc doesn't depend on b.inc nor on itself, etc. The standard tree display doesn't do this:

+-.
  +-SConstruct
  +-a.inc
  +-b.inc
  +-f.out
  | +-f1.in
  | +-f2.in
  | +-a.inc
  | +-b.inc
  +-f1.in
  +-f2.in

[mwichmann]

hmmm, wonder if this is due to the Fake scanner... may need to be a little bit more clever on that.