Add matchspec for foo/bar branch names in git completion

[Daniel Shahaf <d.s@daniel.shahaf.name>]

I'd like to add a matchspec to git heads completion, such that
'git checkout o/m<TAB>' would complete to 'origin/master'.

However, I can't seem to get the matchspec right.  I tried this:

[[[
diff --git a/Completion/Unix/Command/_git b/Completion/Unix/Command/_git
index 9304fbb..fe03439 100644
--- a/Completion/Unix/Command/_git
+++ b/Completion/Unix/Command/_git
@@ -5585,7 +5585,7 @@ __git_remote_branch_names () {
   branch_names=(${${(f)"$(_call_program remote-branch-refs git for-each-ref --format='"%(refname)"' refs/remotes 2>/dev/null)"}#refs/remotes/})
   __git_command_successful $pipestatus || return 1
 
-  _wanted remote-branch-names expl 'remote branch name' compadd "$@" -a - branch_names
+  _wanted remote-branch-names expl 'remote branch name' compadd -M 'r:|/=*' "$@" -a - branch_names
 }
 
 (( $+functions[__git_remote_branch_names_noprefix] )) ||
@@ -5596,7 +5596,7 @@ __git_remote_branch_names_noprefix () {
   branch_names=(${${${(f)"$(_call_program remote-branch-refs-noprefix git for-each-ref --format='"%(refname)"' refs/remotes 2>/dev/null)"}##*/}:#HEAD})
   __git_command_successful $pipestatus || return 1
 
-  _wanted remote-branch-names-noprefix expl 'remote branch name' compadd "$@" -a - branch_names
+  _wanted remote-branch-names-noprefix expl 'remote branch name' compadd -M 'r:|/=*' "$@" -a - branch_names
 }
 
 (( $+functions[__git_commit_objects_prefer_recent] )) ||
@@ -5637,7 +5637,7 @@ __git_heads_local () {
     [[ -f $gitdir/refs/stash ]] && heads+=stash
   fi
 
-  _wanted heads-local expl "local head" compadd "$@" -a - heads
+  _wanted heads-local expl "local head" compadd -M 'r:|/=*' "$@" -a - heads
 }
 
 (( $+functions[__git_heads_remote] )) ||
@@ -5647,7 +5647,7 @@ __git_heads_remote () {
 
   heads=(${(f)"$(_call_program headrefs git for-each-ref --format='"%(refname:short)" refs/remotes' 2>/dev/null)"})
 
-  _wanted heads-remote expl "remote head" compadd "$@" -a - heads
+  _wanted heads-remote expl "remote head" compadd -M 'r:|/=*' "$@" -a - heads
 }
 
 (( $+functions[__git_commit_objects] )) ||
]]]

Now, it does complete 'o/m' to 'origin/master', but when I type just
'or<TAB>', it completes to 'origin', without the trailing slash:

[[[
% git branch -l
  foo/a
  foo/b
  foo/c
* master
% git co fo<TAB>
% git co foo
]]]

How could I get 'fo<TAB>' to become 'foo/', with the slash?

This is both in my usual config and in 'zsh -f'+compinit.

Cheers,

Daniel