Holmake improvements (bug in caching of dep. analysis; diag change)

Two intertwined patches (bad form I know):
1. Fix the bug in tests/indepchildren.
2. Revise the internal diag function to take a function generating a
string rather than a string; then the work of generating the string
only happens if diagnostics have been asked for.

Test-case illustrating a current bug in Holmake

Holmake at the top of (a clean) indepchildren tree will fail to build
ultimateTheory because dir2Theory will not load, not having the
correct dependency on baseTheory recorded in its .uo file.

Not in the build sequence because I don't have a fix for it yet.