Keyword expansion without modifying the file contents should not update file timestamps

When the svn:keywords property is set for a file and the keyword expansion is
performed, the file will currently get a new timestamp, even if the file
contents is not changed at all.
The problem was discovered first after changing the Subversion client from 1.7.6
to 1.9.0 (1.9.1) using the “svn switch” command after creating a tag. In our
specific case the timestamps of all files with the svn:keywords “Author Date
Revison” are updated, although there was change in the file contents at all. Our
gmake based build system now has to compile more or less everything.
	
After a discussion with Stefan Sperling on users@subversion.apache.org he
confirmed that the behavior is a side effect of fixing Issue #1975.

I suggest the following change:

During the keyword  translation process (in several commands) a file should only
be modified, if the contents of the file really changes. Using the “svn switch”
command a change would only occur, if the keywords $URL$ and $Header$ are used.

Original issue reported by dwolfram