Use Pathname::cleanpath to fix up broken prefixes (as Ancor suggested)

src/lib/installation/unmounter.rb

@@ -13,6 +13,7 @@
 # ------------------------------------------------------------------------------
 
 require "yast"
+require "pathname"
 
 module Installation
   # Class to handle unmounting all mounts from the given subtree on in the
@@ -74,8 +75,7 @@ def to_s
     #   read_mounts_file() or add_mount().
     #
     def initialize(mnt_prefix = "/mnt", mounts_file_name = "/proc/mounts")
-      @mnt_prefix = mnt_prefix.dup
-      @mnt_prefix.chomp!("/") unless @mnt_prefix == "/"
+      @mnt_prefix = Pathname.new(mnt_prefix).cleanpath.to_s
       clear
       read_mounts_file(mounts_file_name) unless mounts_file_name.nil?
     end

test/unmounter_test.rb

@@ -57,9 +57,9 @@ def mount(mount_path)
       expect(um.mnt_prefix).to eq "/foo"
     end
 
-    it "does not overdo insanely broken prefixes" do
-      um = described_class.new("/foo//", nil)
-      expect(um.mnt_prefix).to eq "/foo/"
+    it "even fixes up insanely broken prefixes" do
+      um = described_class.new("/foo///bar///", nil)
+      expect(um.mnt_prefix).to eq "/foo/bar"
     end
 
     it "leaves a root directory prefix as it is" do