test-svn-fe: split off "test-svn-fe -d" into a separate function

test-svn-fe: split off "test-svn-fe -d" into a separate function

The helper for testing the svndiff library is getting dangerously
close to the right margin.  Split it off into a separate function so
it is easier to contemplate on its own.

In the process, make the test_svnfe_usage[] string static so it can be
shared by the two functions (and other future functions in this test
program) without fuss.

In other words, this just unindents the code a little.  No functional
change intended.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>