When creating a diff with Arcanist in an SVN repository which contains modifications to both current-level repository files and external repository files a usage exception is thrown for mismatched base revisions. Reverting the files avoids the exception.
Here is a cleansed and snipped log.
```
$ arc diff
You have untracked files in this working copy.
Working copy: /myPjt/
Untracked files in working copy:
.cproject
.project
.settings
Since you don't have 'svn:ignore' rules for these files, you may have
forgotten to 'svn add' them.
Do you want to add these files to the commit? [y/N]
You have not specified any reviewers. Continue anyway? [y/N] y
Linting...
No lint engine configured for this project.
Running unit tests...
No unit test engine is configured for this project.
The working copy includes changes to 'svn:externals' paths. These changes will not be included in the diff because SVN can not commit 'svn:externals' changes alongside normal changes.
Modified 'svn:externals' files:
libs/canopen/pjt/engine/engine.mk
libs/canopen/pjt/engine/genqmake.mk
Generate a diff (with just local changes) anyway? [y/N] y
Usage Exception: Base revisions of changed paths are mismatched. Update all paths to the same base revision before creating a diff:
Revision 203, .
Revision 203, inc/App_Task.h
Revision 154, libs/canopen/pjt/engine/engine.mk
Revision 154, libs/canopen/pjt/engine/genqmake.mk
$ svn propget svn:externals .
^/../db_svn_bios/trunk/canopen_library@260 libs/canopen
```