fix: default goal in makefile and add goal for unit tests

[xperimental]

Three changes, but all in the Makefile:

• Fix git command to ignore external diff tools
• Fix definition of default target
• Add a target for unit tests (optional, but I like being able to run tests with a simple command)

I can split this up into separate PRs, if you like.

[jan--f]

Afaiu the .DEFaULT_GOAL variable is intended for more complex use cases. The make docs say

The .DEFAULT_GOAL variable allows you to discover the current default goal, restart the default goal selection algorithm by clearing its value, or to explicitly set the default goal.

I don't a benefit over just letting make pick the first target as the default.

[xperimental]

The advantage of .DEFAULT_GOAL is that you explicitly set the default goal instead of relying on the structure of the Makefile to define the default goal. For example, in this Makefile we include another file above this declaration, which already defines some goals of which one becomes the default goal.

We can either move the goal we want as default further up in the file (above the include, in this case) or define what we want explicitly (which is what I opted for).

[jan--f]

Fair point, I didn't think of the import.

[jan--f]

/lgtm
/approve

[openshift-ci]

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