[llvm][docs] Add step by step git to GettingStarted

[zbrid]

Summary

Due to deleting the git llvm script, folks were asking for better documentation
about how to use git in order to commit to the Github repo. I added some step
by step git commands to make the usage clearer.

Context link: http://lists.llvm.org/pipermail/llvm-dev/2020-May/141640.html

[zbrid]

Add running tests to the workflow

[mehdi_amini]

llvm/docs/GettingStarted.rst

511

This isn't super useful I believe, this only shows for example something like:

$ git push gh HEAD:master --dry-run
To github.com:llvm/llvm-project.git
   ffc6e593d271..e88c68267154  HEAD -> master

What about instead:

$ git log origin/master...HEAD --oneline

which shows the list of commits about to be pushed?

[zbrid]

Change the dry-run command to mehdi's suggestion

[zbrid]

Good idea. Done.

[spatel]

llvm/docs/GettingStarted.rst

511

Thanks - that's basically the feature of 'git llvm push' that was most useful to me.
It did some variation of this:

$ git llvm push
`git fetch https://github.com/llvm/llvm-project.git master` printed to stderr:
From https://github.com/llvm/llvm-project
 * branch                    master     -> FETCH_HEAD
Pushing 2 commits:
  f29a9387ace [x86] add tests for disguised horizontal ops; NFC
  f40b5065511 [x86] add tests for heroic horizontal ops; NFC
Are you sure you want to create 2 commits? (y/N): n

That 'Are you sure...' pause is what kept me out of trouble, so I'd probably just end up creating a weak version of the old script again.

[zbrid]

@spatel - I updated the documentation here to use the command mehdi suggested (that you said was good in a follow up comment). The rest of it look good too?

[spatel]

In D80088#2042918, @zbrid wrote:

@spatel - I updated the documentation here to use the command mehdi suggested (that you said was good in a follow up comment). The rest of it look good too?

Sure; I don't know if there's more that could be added, but this LGTM.

llvm/docs/GettingStarted.rst

 
 Once a patch is reviewed, you should rebase it, re-test locally, and commit the
 changes to LLVM's master branch. This is done using `git push` if you have the
 required access rights. See `committing a change
 <Phabricator.html#committing-a-change>`_ for Phabricator based commits or
 `obtaining commit access <DeveloperPolicy.html#obtaining-commit-access>`_
 for commit access.
 
+Here is an example workflow using git. This workflow assumes you have an
+accepted commit on the branch named `branch-with-change`.
+
+.. code-block:: console
+
+  # Go to the branch with your accepted commit.
+  % git checkout branch-with-change
+  # Rebase your change onto the latest commits on Github.
+  % git pull --rebase origin master
+  # Rerun the appropriate tests if needed.
+  % ninja check-$whatever
+  # Check that the intended push will do the right thing.
+  % git push origin HEAD:master --dry-run
+  # Push to Github.
+  % git push origin HEAD:master
+
 LLVM currently has a linear-history policy, which means that merge commits are
 not allowed. The `llvm-project` repo on github is configured to reject pushes
 that include merges, so the `git rebase` step above is required.
 
+Please ask for help if you're having trouble with your particular git workflow.
+
 Bisecting commits
 ^^^^^^^^^^^^^^^^^
 
 See `Bisecting LLVM code <GitBisecting.html>`_ for how to use ``git bisect``
 on LLVM.
 
 Reverting a change
 ^^^^^^^^^^^^^^^^^^