gitcmd: Pass in configs to enable custom diff algorithms

Pass in a set of configs that is of the form

[diff "<name>"]
  algorithm = <name>

This allows adding the following to .gitattributes to set diff algorithms based on a path:

*.json diff=<name>

meyers, patience, histogram, minimal can be used as

This utilizes the ability in Git to set the internal diff algorithm

To test

In order to test, add a file with before and after contents with the following:

before:

unique line 1
common
common
common
unique line 2
common
common
common
unique line 3

after:

unique line 3
common
common
common
unique line 1
common
common
common
unique line 2

In your GDK, add a .gitattributes file in the default branch:

*.txt diff=histogram

*.txt diff=myers

When you navigate to the MR diffs, the diffs should appear different.

fixes: #2591