Add and use warn() routine

dotfiles/gittum/bin/remote-edit

 # Edit URL of remote $1 using vipe(1)
 #
 
+warn() {
+    echo "$@" >&2
+}
+
 die() {
-    echo "$@" 1>&2
+    warn "$@"
     exit 1
 }
 
 usage() {
-    echo "usage: git $(basename "$0") [-v|--verbose] REMOTE"
+    warn "usage: git $(basename "$0") [-v|--verbose] REMOTE"
     exit 0
 }