Abortable merge, clone, and checkout commands

[nelsonni]

Is your feature request related to a problem? Please describe.
Long running processes should be cancelable by the user. For version control systems, the commands that can become long-running are merge, clone, and checkout; the long-running nature of these commands stems from accessing and downloading files from remote repositories.

The need for cancelling asynchronous tasks was identified shortly after Promise was added to the ES2015 standard. However, the AbortController option that was introduced only for the fetch() command and needs to be manually added to other commands (per the article, Aborting a signal: How to cancel an asynchronous task in JavaScript).

Describe the solution you'd like
Since merge, clone, and checkout rely on isomorphic-git, we need to wait for the introduction of isomorphic-git/isomorphic-git#867.

Describe alternatives you've considered
Adding AbortController and AbortSignal parameters to the merge, clone, and checkout functions, which already wrap around the comparable functions in isomorphic-git. However, this option would not be able to interject an abort into the isomorphic-git portion of the process, which contain the potentially long-running code, and therefore only allow for aborting the additional paths wrapped around them.

Additional context
N/A