git-branch-list(is_a_branch): Remove `$?` from `return`

I had forgotten when I wrote this, but the Bash help says:

> If N is omitted, the return status is that of the last command.

Meaning we don't need to explicitly return the code from the last
command as that's what happens by default.

1 files changed, 1 insertions, 1 deletions

diff --git a/git-branch-list b/git-branch-list
index 10713df..e7d7dfe 100755
--- a/git-branch-list
+++ b/git-branch-list
@@ -88,7 +88,7 @@ function is_a_branch () {
 	local branch="$1"
 
 	git rev-parse --verify "$branch" > /dev/null 2>&1
-	return $?
+	return
 }
 
 function branch_by_id () {