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/" may themselves reference foreign
commits.

Currently, we break in this case and can't find the commit reference. Instead,
generate a foreign commit stub so we can at least point at some reasonable
object.

Test Plan: Successfully imported trunk/a/ of the test repo in T325 without
errors. Verified commit 3 in that repo is imported as a foreign stub.

Reviewers: jungejason, nh, tuomaspelkonen, aran

Reviewed By: jungejason

CC: aran, jungejason, epriestley

Differential Revision: 892