Autotest: ChangeLog search expensive

[Ralf Wildenhues]

The Autotest testsuite startup code searching for ChangeLog files below
the source tree is potentially very expensive.  Not sure what the best
way out is here, maybe some way to allow the developer to set a fixed
string at `make dist' time, or allow to try out some version control
command.  But say, searching the disk for half a minute due to cold
cache, isn't such a great idea.

Cheers,
Ralf