hg.clone: do not ignore branch argument when source is a repo object

This is a revert of 4a70178f9bde. The "bug" mentioned in this changeset is unclear:
hopefully using a test to cover this usage should prevent any bugs.

mercurial/hg.py

@@ -221,7 +221,7 b' def clone(ui, source, dest=None, pull=Fa'
         src_repo = repository(ui, source)
     else:
         src_repo = source
-        branch = (None, [])
+        branch = (None, branch or [])
         origsource = source = src_repo.url()
     rev, checkout = addbranchrevs(src_repo, src_repo, branch, rev)
 

tests/test-clone

@@ -209,4 +209,14 b' EOF'
 python simpleclone.py
 rm -r ua
 
+cat <<EOF > branchclone.py
+from mercurial import ui, hg
+myui = ui.ui()
+repo = hg.repository(myui, 'a')
+hg.clone(myui, repo, dest="ua", branch=["stable",])
+EOF
+
+python branchclone.py
+rm -r ua
+
 exit 0

tests/test-clone.out

@@ -314,3 +314,10 b' e8ece76546a6'
 e8ece76546a6
 updating to branch default
 3 files updated, 0 files merged, 0 files removed, 0 files unresolved
+requesting all changes
+adding changesets
+adding manifests
+adding file changes
+added 14 changesets with 14 changes to 3 files
+updating to branch stable
+3 files updated, 0 files merged, 0 files removed, 0 files unresolved