convert/svn: make sink recover gracefully from empty changeset
Otherwise when processing a changeset that in fact changes no files
(perhaps due to bug in import from CVS) can get something like:
unexpected svn output:
abort: unable to cope with svn output
Bug report and patch draft by Jesse Glick <jesse.glick@sun.com>
adding a
% default context
diff -r cf9f4ba66af2 a
--- a/a
+++ b/a
@@ -2,7 +2,7 @@
c
a
a
-b
+dd
a
a
c
% invalid --unified
abort: diff context lines count must be an integer, not 'foo'
% --unified=2
diff -r cf9f4ba66af2 a
--- a/a
+++ b/a
@@ -3,5 +3,5 @@
a
a
-b
+dd
a
a
% diff.unified=2
diff -r cf9f4ba66af2 a
--- a/a
+++ b/a
@@ -3,5 +3,5 @@
a
a
-b
+dd
a
a
% diff.unified=2 --unified=1
diff -r cf9f4ba66af2 a
--- a/a
+++ b/a
@@ -4,3 +4,3 @@
a
-b
+dd
a
% invalid diff.unified
abort: diff context lines count must be an integer, not 'foo'
% test off-by-one error with diff -p
adding a
diff -r cb9a9f314b8b a
--- a/a
+++ b/a
@@ -1,0 +1,1 @@
+b
@@ -2,0 +3,1 @@ a
+c