Steps to reproduce:
1. Create svn externals file in the working copy of a check out project using 'svn propedit svn:externals .' command
2. Enter a line with <local path>/<local file name> http://<svn url>/<svn file name>, where:
<local path> is the name of the local directory where the file should be check out
<local file name> is the name of the file
http://<svn url> is snv location
<svn file name> is the name of the file in svn
4. Save and commit the svn externals file.
5. Create Jenkins project and configure it to check out the svn url where you have commit the svn externals.
6. Optionally do a fresh check out on the machine using the native svn client version: 1.6.17 (r1128011) to ensure that the check out of the externals is working.

Result: Jenkins Subversion plugin fails to check out files specified in snv:externals

Error log:

ERROR: Failed to parse svn info for external http://<svn url>/<svn file name> at .///<local dir>/<local file name>
org.tmatesoft.svn.core.SVNException: svn: '<local path>/./<local dir>/<local file name>' is not a working copy
at org.tmatesoft.svn.core.internal.wc.SVNErrorManager.error(SVNErrorManager.java:64)
at org.tmatesoft.svn.core.internal.wc.admin.SVNWCAccess.probeOpen(SVNWCAccess.java:316)
at org.tmatesoft.svn.core.internal.wc.admin.SVNWCAccess.probeOpen(SVNWCAccess.java:295)
at org.tmatesoft.svn.core.wc.SVNWCClient.crawlEntries(SVNWCClient.java:3304)
at org.tmatesoft.svn.core.wc.SVNWCClient.doInfo(SVNWCClient.java:2506)
at org.tmatesoft.svn.core.wc.SVNWCClient.doInfo(SVNWCClient.java:2873)
at hudson.scm.SubversionSCM$BuildRevisionMapTask.invoke(SubversionSCM.java:1038)
at hudson.scm.SubversionSCM$BuildRevisionMapTask.invoke(SubversionSCM.java:1008)
at hudson.FilePath.act(FilePath.java:783)
at hudson.FilePath.act(FilePath.java:765)
at hudson.scm.SubversionSCM.checkout(SubversionSCM.java:692)
at hudson.model.AbstractProject.checkout(AbstractProject.java:1195)
at hudson.model.AbstractBuild$AbstractRunner.checkout(AbstractBuild.java:568)
at hudson.model.AbstractBuild$AbstractRunner.run(AbstractBuild.java:457)
at hudson.model.Run.run(Run.java:1404)
at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:46)
at hudson.model.ResourceController.execute(ResourceController.java:88)
at hudson.model.Executor.run(Executor.java:238)
Caused by: org.tmatesoft.svn.core.SVNErrorMessage: svn: '<local path>/./<local dir>/<local file name>' is not a working copy
at org.tmatesoft.svn.core.SVNErrorMessage.create(SVNErrorMessage.java:163)
at org.tmatesoft.svn.core.SVNErrorMessage.create(SVNErrorMessage.java:118)
at org.tmatesoft.svn.core.internal.wc.admin.SVNWCAccess.probeOpen(SVNWCAccess.java:315)
... 16 more