difftree should ignore changes in permissions between the old, deprecated, group writable and ordinary files

[alcortesm]

On 2005 the set of modes supported by regular files in git included:

1. 100755 = rwx-r-xr-x (regular file, executable)
2. 100644 = rw-r--r-- (regular file, non-executable)
3. 100664 = rw-rw-r-- (regular file, non-executable, group writable)

Nowadays only the first two are supported (https://github.com/gitster/git/blob/master/Documentation/technical/index-format.txt).

But there are still repositories around with files with the old deprecated 100664 mode.

When git diff-tree see that a file has change its mode from 100664 to 100644, it ignores the change. But go-git is not ignoring it, we should fix that.

An example of such behavior can be found in git/git@5c2a7fb where three changes were introduced:

• Makefile contents was modified.
• read-cache.c contents was modified and its permissions where changed from 100664 to 100644.
• update-cache.c permissions were changed from 100664 to 100644 (github does not show this).

If you run git difftree over this commit and its parent, the update-cache.c file is ignored:

; git diff-tree -r bdd4da595a265a091a859754a22688f7351cddee..5c2a7fbc362e4227ced84c32c3fdc9682d085962
:100644 100644 c7e5c7467d603fcaad63ee2bcade0889c21015d8 ac61a61b0e99203bd757b8aab7e11fc550197855 M	Makefile
:100644 100644 f1abae1442e72753dc8356bcbdc860be9a645bc4 2ee96bc92c37a204a844afb3354483b733db0c4e M	read-cache.c

But go-git difftree reports update-cache.c was also modified.