cmMakefile: Convert to local paths in IssueMessage

This is the only place we care show the FilePath to the user, so defer
the expensive relative path calculation until here.
cmListFileBacktrace: Add a MakeRelative method

This allows deferring backtraces until the moment when a relative path
is actually necessary. Unfortunately, not only cmMakefile issues
messages, so any caller of cmake::IssueMessage has to make the backtrace
relative as well.
Merge topic 'dev/backtrace-performance' into next

4ded1520 cmListFileBacktrace: Add a MakeRelative method
b8144963 cmMakefile: Convert to local paths in IssueMessage