cat: default to working dir parent instead of tip
This introduces a defaultrev function that chooses
the working dir parent if a revision isn't specified,
and uses it in several places.
diff -r acd8075edac9 b
--- /dev/null
+++ b/b
@@ -0,0 +1,1 @@
+123
diff -r acd8075edac9 b
--- /dev/null
+++ b/b
@@ -0,0 +1,1 @@
+123
diff -r acd8075edac9 a
--- a/a
+++ b/a
@@ -0,0 +1,1 @@
+foo
diff -r acd8075edac9 b
--- /dev/null
+++ b/b
@@ -0,0 +1,1 @@
+123