Enable user to override default diff -M arg

[mellowed100]

Small fix to allow overriding of default diff -M arg. Using --find-renames= or -M or --no-renames will disable -M.

Full discussion is here:
#1195

[Byron]

That looks simple enough, thanks a lot!

Is it possible to have a test leverage this newly gained capability?
Now it's possible to do that, previously it would generate an invalid command-line, so a test can probably check for a successful diff call.

[mellowed100]

Let me know if this works for you. thanks!

[Byron]

This looks great already!

I left a comment with a tiny last request to complete the M journey with a show case on how callers can now use it themselves :).

[mellowed100]

Hey Byron!
So I updated the test to address those changes you asked for. The initial commits of file_a.txt and file_b.txt have been modified to have a similarity index of 52%. So the default diff -M (50%) should still flag it as a renamed file. I added diff -M75% and diff -M40% to check for both added/deleted files and renamed files. Let me know if this works for you. Thanks!

[Byron]

Thanks a lot - doing it like this is fantastic and I couldn't imagine a way to better show the effect of the M flag.

I will merge once CI runs through. Silly me aborted the existing Cygwin run because it seemed stuck, but history shows it's taking between 13 and 19 minutes to run there - incredible especially since the linux hosts are finished in less than 3 minutes.

[mellowed100]

You're welcome! Thanks for your guidance and patience (This was my first PR). I really appreciate it! -Take care!