Initial "b4 diff" landed in master

Initial "b4 diff" landed in master

[Konstantin Ryabitsev]

Hi, all:

Borrowing liberally from Jason's jg_tools, I had a first go at 
implementing "b4 diff". It's pretty slick when it works, for which it 
currently needs to be able to find the exact indexes for the files being 
modified. I found that having linux-next in my remotes helped to get 
most of those blobs into my local tree.

I would be happy if you poke at it some and let me know how it's working 
for you and what output improvements you can suggest.

Example:

$ b4 diff 1587451187-6889-1-git-send-email-masonccyang@mxic.com.tw
Looking up https://lore.kernel.org/r/1587451187-6889-1-git-send-email-masonccyang%40mxic.com.tw
Grabbing thread from lore.kernel.org/linux-mtd
Retrieved 26 messages in the thread
Checking for older revisions on https://lore.kernel.org/linux-mtd/
Added 17 messages from thread: [PATCH 0/4] mtd: spi-nor: Add support for Octal 8D-8D-8D mode
---
Analyzing 45 messages in the thread
Preparing fake-am for v1: spi: spi-mem: Add support for Octal 8D-8D-8D mode
  range: c20efe82ab72..6083ca557825
Preparing fake-am for v2: mtd: spi-nor: Add support for Octal 8D-8D-8D mode
  range: 5eaa5d76ab2e..8bfd7eea5726
---
Success, you may now run:
    git range-diff c20efe82ab72..6083ca557825 5eaa5d76ab2e..8bfd7eea5726


Best,
-K