tests: don't rely on broken behaviour in test-largefiles-cache.t

The test relied on the bug that 'pull largefiles from branchheads' didn't pull
any largefiles from tip revision when it seemed like no largefiles had been
checked out before.

tests/test-largefiles-cache.t

@@ -16,6 +16,9 b' Create source repo, and commit adding la'
   $ echo large > large
   $ hg add --large large
   $ hg commit -m 'add largefile'
+  $ hg rm large
+  $ hg commit -m 'branchhead without largefile'
+  $ hg up -qr 0
   $ cd ..
 
 Discard all cached largefiles in USERCACHE
@@ -35,7 +38,7 b' repo as "default" path in .hg/hgrc.'
   adding changesets
   adding manifests
   adding file changes
-  added 1 changesets with 1 changes to 1 files
+  added 2 changesets with 1 changes to 1 files
   (run 'hg update' to get a working copy)
   caching new largefiles
   0 largefiles cached
@@ -44,7 +47,7 b' Update working directory to "tip", which'
 but there is no cache file for it.  So, hg must treat it as
 "missing"(!) file.
 
-  $ hg update
+  $ hg update -r0
   getting changed largefiles
   error getting id 7f7097b041ccf68cc5561e9600da4655d21c6d18 from url file:$TESTTMP/mirror for file large: can't get file locally (glob)
   0 largefiles updated, 0 removed
@@ -61,7 +64,7 b' Update working directory to null: this c'
 
 Update working directory to tip, again.
 
-  $ hg update
+  $ hg update -r0
   getting changed largefiles
   error getting id 7f7097b041ccf68cc5561e9600da4655d21c6d18 from url file:$TESTTMP/mirror for file large: can't get file locally (glob)
   0 largefiles updated, 0 removed
@@ -90,6 +93,7 b' from file in working copy:'
   $ chmod 660 large
   $ echo change >> large
   $ hg commit -m change
+  created new head
   $ ../ls-l.py .hg/largefiles/e151b474069de4ca6898f67ce2f2a7263adf8fea
   640