Use full refname instead of short to prevent disambiguation with tag

In the unlikely scenario that you have a remote branch on `origin` called
`foo`, and a local tag called `origin/foo`, git changes the behavior of
the previous command such that it produces

```
$ git for-each-ref --sort=refname --format=%(refname:short) refs/remotes

origin/branch1
remotes/origin/foo
```

with `remotes/` prepended. Presumably this is to disambiguate it from
the local tag `origin/foo`. Unfortunately, this breaks the existing
behavior of this function, so the remote branch is never shown.

By changing the command, we now get
```
$ git for-each-ref --sort=refname --format=%(refname) refs/remotes

refs/remotes/origin/branch1
refs/remotes/origin/foo
```

This allows easy parsing based on the `/`, and none of the code outside
this function has to change.

----

We previously were not showing remote HEADs for modern git versions
based on how they were formatted from "%(refname:short)".
We have decided that this is a feature, not a bug, so we are building
that into the code here.

Author: Chris McDonnell

pkg/commands/git_commands/remote_loader.go

@@ -96,19 +96,23 @@ func (self *RemoteLoader) getRemoteBranchesByRemoteName() (map[string][]*models.
 
 	cmdArgs := NewGitCmd("for-each-ref").
 		Arg(fmt.Sprintf("--sort=%s", sortOrder)).
-		Arg("--format=%(refname:short)").
+		Arg("--format=%(refname)").
 		Arg("refs/remotes").
 		ToArgv()
 
 	err := self.cmd.New(cmdArgs).DontLog().RunAndProcessLines(func(line string) (bool, error) {
 		line = strings.TrimSpace(line)
 
-		split := strings.SplitN(line, "/", 2)
-		if len(split) != 2 {
+		split := strings.SplitN(line, "/", 4)
+		if len(split) != 4 {
+			return false, nil
+		}
+		remoteName := split[2]
+		name := split[3]
+
+		if name == "HEAD" {
 			return false, nil
 		}
-		remoteName := split[0]
-		name := split[1]
 
 		_, ok := remoteBranchesByRemoteName[remoteName]
 		if !ok {