R1:ae045a9cf266

When doing partial subdirectory parses in Subversion, stub out foreign commit
references

Summary:
See T325. We tentatively support doing partial subdirectory parses in
Phabricator for Subversion, so you can elect to import only "trunk/local/" or
similar. We do this by importing only some of the commits (those commits which
affected that directory).

In Subversion, you can also "svn cp
svn+ssh://example.com/svnroot/trunk/foreign/example.c@13 local.c". This means
that commits which reference "trunk/local/"…