Always pass commit message to `git::create_derived_commit`

This function used to take an `Option<&str>` as commit message and provided a default. However, there was only one place where we passed `None`, so it's cleaner to just always pass the message.
We used to trim the message and append a new line at the end. However, `git2` provides a function to clean up a commit message, which is used here now.

Test Plan: `cargo build` and use the fresh build to submit this Pull Request

Reviewers: jozef-mokry

Reviewed By: jozef-mokry

Pull Request: #32

Always pass --no-verify to git push

We have git hooks in our main development repository, which run extensive tests on the current work tree before push. It does not always make sense to run those from `spr` since what spr pushes is often not the current worktree.
A better, future solution would be to have a configurable pre-submit/pre-land tests implemented in spr itself.

Test Plan:
update and see that `spr diff` and `spr land` work much quicker in our main repo

Reviewers: AlbertQM, jozef-mokry

Reviewed By: AlbertQM

Pull Request: #29

Move asynchronous memoization into generic class and simplify GitHub …

…class

We nicely optimise requesting information from GitHub, but it came with a lot of repetetive complicated code in the GitHub class. This commit moves that into a separate, generic class and adds unit tests to it, proving that memoization works and that execution is not sequential but parallel.

Test Plan: `cargo test`, submitting this commit using `spr diff`

Reviewers: jozef-mokry

Reviewed By: jozef-mokry

Pull Request: #12

Reword message when creating a non-stacked Pull Request

If you only want to submit a Pull Request for review, you might be shocked to see the landing plane and talk of landing. This should clarify a little.

Test Plan:
build and run `spr diff` to submit thic commit as a PR and see this message

Reviewers: smhmd

Reviewed By: smhmd

Pull Request: #10

After landing a Pull Request, delete the old branch on GitHub

GitHub may do this automatically, if it's configured that way, but it doesn't hurt to just try do it. The exit code from 'git push' is ignored anyway.

Test Plan:
Land a PR with `spr land` in a repo that is not configured to automatically delete PR branches.

Reviewers: jozef-mokry

Reviewed By: jozef-mokry

Pull Request: #2

Rebase local branch after landing

Initial version