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>