shows diff in terminal

[agzam]

if gh is installed, shows the diff in the terminal (instead of browser)

Replace this with a summary of what you've changed and why, followed by a list of issues it affects, if any.

Fix: #0000
Ref: #0000
Close: #0000

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• I searched the issue tracker and this hasn't been PRed before.
• My changes are not on the do-not-PR list for this project.
• My commits conform to Doom's git conventions.
• My changes are visual; I've included before and after screenshots.
• I am blindly checking these off.
• Any relevant issues or PRs have been linked to.
• This a draft PR; I need more time to finish it.

[hlissner]

The direction I want to take with doom upgrade is to display (or link to) portions of an org changelog (which won't exist until v3), rather than display raw git diffs, so I must turn this down. Thanks for the PR and for bringing it to my attention, in any case!