Fix renaming files during an amendment

Commit 7f23eb4 introduced support for including modifications from the
last commit when amending. Unfortunately, this introduces a new case
where renaming a file as part of an amendment would result in a crash as
Overcommit would try to update the modification time of the non-existent
old file.

This also had the nasty effect of losing the user's changes, which is a
regression that was introduced in c3d972d. I'll be addressing that in a
separate commit.

A simple workaround here is to simply not bother updating the
modification time if the file does not exist.

Note that `File.mtime` sometimes behaves strangely here--I've seen it
return `nil` for files that don't exist, but raise errors at other times
(and I have no idea under which conditions one outcome occurs versus the
other).

It's easiest to just check if the file exists before we call `mtime`.

Author: sds

lib/overcommit/hook_context/pre_commit.rb

@@ -180,6 +180,7 @@ def store_modified_times
 
       (staged_files + unstaged_files).each do |file|
         next if Overcommit::Utils.broken_symlink?(file)
+        next unless File.exist?(file) # Ignore renamed files (old file no longer exists)
         @modified_times[file] = File.mtime(file)
       end
     end

spec/overcommit/hook_context/pre_commit_spec.rb

@@ -150,6 +150,29 @@
       end
     end
 
+    context 'when renaming a file during an amendment' do
+      around do |example|
+        repo do
+          `git commit --allow-empty -m "Initial commit"`
+          `touch some-file`
+          `git add some-file`
+          `git commit -m "Add file"`
+          `git mv some-file renamed-file`
+          example.run
+        end
+      end
+
+      before do
+        context.stub(:amendment?).and_return(true)
+      end
+
+      it 'does not try to update modification time of the old non-existent file' do
+        File.should_receive(:mtime).with(/renamed-file/)
+        File.should_not_receive(:mtime).with(/some-file/)
+        subject
+      end
+    end
+
     context 'when only a submodule change is staged' do
       around do |example|
         submodule = repo do