DiffParser: ignore chmod change

[Luolc]

Taken from #22 (comment)

If the only change is a chmod(no changes to the contents of the file), we should ignore the change altogether.

After this if fixed, the UT of only chmod change in #22 should update.

We will need a new test for this: chmod plus file change.

[Luolc]

Some experiment:

$ git init
$ echo hello > HelloWorld
$ git add .
$ git commit -m "initial commit"
[master (root-commit) 83e0652] initial commit
 1 file changed, 1 insertion(+)
 create mode 100644 HelloWorld
$
$ git checkout -b foo
Switched to a new branch 'foo'
$ chmod 755 HelloWorld
$ git status
On branch foo
Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git checkout -- <file>..." to discard changes in working directory)

  modified:   HelloWorld

no changes added to commit (use "git add" and/or "git commit -a")
$ git add .
$ git commit -m "change filemode"
[foo f3c937d] change filemode
 1 file changed, 0 insertions(+), 0 deletions(-)
 mode change 100644 => 100755 HelloWorld
$
$ git diff master
diff --git a/HelloWorld b/HelloWorld
old mode 100644
new mode 100755
$ git config core.filemode false
$ git diff master
diff --git a/HelloWorld b/HelloWorld
old mode 100644
new mode 100755
$
$ chmod 644 HelloWorld
$ git status
On branch foo
nothing to commit, working tree clean

So we can't ignore filemode change after a mode change is already committed. core.filemode could only works for detecting new changes, but not for showing existed diff. The only way I could find now is checking this by the size of "edit lines". If the size is zero, then this is only a filemode change.

IMO, we'd better work on this after we introducing line changes in the future.