Has anyone written  a script like this that invokes the install
program only if the source contents != the target contents.

Does anyone think this would/wouldn't be a good patch to the install program?

I think this is a fairly common idiom and useful in both build environments,
as well as mtree/tripwire-style systems (the two environs I can think of
that really use mtimes).

I've written (well, stolen) "cp-if-change" and "mv-if-change" from some
GNU tools.  Trivial scripts, really, but they don't have the full
capabilities of their /bin counterparts (can't cp multiple files into
a dir, etc).