Store empty (default) branch in branch cache, too.
Operations on this branch don't work otherwise.
Reading branches.cache had to be adjusted to allow an empty label.
Adjusted reading of the cache tip for symmetry, no functional change here.
% init
% commit
adding 1/base
adding 2/base
% qnew mqbase
% qrefresh
% qdiff
diff -r b55ecdccb5cf 1/base
--- a/1/base
+++ b/1/base
@@ -1,1 +1,1 @@ base
-base
+patched
diff -r b55ecdccb5cf 2/base
--- a/2/base
+++ b/2/base
@@ -1,1 +1,1 @@ base
-base
+patched
% qdiff dirname
diff -r b55ecdccb5cf 1/base
--- a/1/base
+++ b/1/base
@@ -1,1 +1,1 @@ base
-base
+patched
diff -r b55ecdccb5cf 2/base
--- a/2/base
+++ b/2/base
@@ -1,1 +1,1 @@ base
-base
+patched
% patch file contents
mqbase
diff -r b55ecdccb5cf 1/base
--- a/1/base
+++ b/1/base
@@ -1,1 +1,1 @@ base
-base
+patched
diff -r b55ecdccb5cf 2/base
--- a/2/base
+++ b/2/base
@@ -1,1 +1,1 @@ base
-base
+patched
% qrefresh 1
% qdiff
diff -r b55ecdccb5cf 1/base
--- a/1/base
+++ b/1/base
@@ -1,1 +1,1 @@ base
-base
+patched
diff -r b55ecdccb5cf 2/base
--- a/2/base
+++ b/2/base
@@ -1,1 +1,1 @@ base
-base
+patched
% qdiff dirname
diff -r b55ecdccb5cf 1/base
--- a/1/base
+++ b/1/base
@@ -1,1 +1,1 @@ base
-base
+patched
diff -r b55ecdccb5cf 2/base
--- a/2/base
+++ b/2/base
@@ -1,1 +1,1 @@ base
-base
+patched
% patch file contents
mqbase
diff -r b55ecdccb5cf 1/base
--- a/1/base
+++ b/1/base
@@ -1,1 +1,1 @@ base
-base
+patched
% qrefresh . in subdir
% qdiff
diff -r b55ecdccb5cf 1/base
--- a/1/base
+++ b/1/base
@@ -1,1 +1,1 @@ base
-base
+patched
diff -r b55ecdccb5cf 2/base
--- a/2/base
+++ b/2/base
@@ -1,1 +1,1 @@ base
-base
+patched
% qdiff dirname
diff -r b55ecdccb5cf 1/base
--- a/1/base
+++ b/1/base
@@ -1,1 +1,1 @@ base
-base
+patched
diff -r b55ecdccb5cf 2/base
--- a/2/base
+++ b/2/base
@@ -1,1 +1,1 @@ base
-base
+patched
% patch file contents
mqbase
diff -r b55ecdccb5cf 1/base
--- a/1/base
+++ b/1/base
@@ -1,1 +1,1 @@ base
-base
+patched
% qrefresh in hg-root again
% qdiff
diff -r b55ecdccb5cf 1/base
--- a/1/base
+++ b/1/base
@@ -1,1 +1,1 @@ base
-base
+patched
diff -r b55ecdccb5cf 2/base
--- a/2/base
+++ b/2/base
@@ -1,1 +1,1 @@ base
-base
+patched
% qdiff dirname
diff -r b55ecdccb5cf 1/base
--- a/1/base
+++ b/1/base
@@ -1,1 +1,1 @@ base
-base
+patched
diff -r b55ecdccb5cf 2/base
--- a/2/base
+++ b/2/base
@@ -1,1 +1,1 @@ base
-base
+patched
% patch file contents
mqbase
diff -r b55ecdccb5cf 1/base
--- a/1/base
+++ b/1/base
@@ -1,1 +1,1 @@ base
-base
+patched
diff -r b55ecdccb5cf 2/base
--- a/2/base
+++ b/2/base
@@ -1,1 +1,1 @@ base
-base
+patched