[JENKINS-18935] Upgrade svnkit library to 1.8.7 and add 1.8 in the global workspace options.
Test unit to verify working copy format.
Fixed test units to get the working copy format.
Jenkins has a hard coded wc format of 100 for svn 1.7.
Workaround for SVNKIT-430. There were two issues fixed as part of this commit:
1. Working copy for 1.7 was being stored as 100. Instead we deprecated the 100 value, and switch to using ISVNWCDb.WC_FORMAT_17. The changes in the global.jelly file are there for maintaining backwards compatibility when upgrading to a new version, but maintaining the 1.7 workspace.
2. Actual workaround for SVNKIT-430. Previous versions used the doCheckout method, however because of SVNKIT-430 the workspace format would default to 1.8. Instead we get the checkout object and add the working copy format manually.
Fixed test unit. Add ignore externals to checkout call.
Merge pull request #103 from christ66/JENKINS-18935

[JENKINS-18935] Add support for subversion 1.8 format