git-terminal.md: add commands to update a fork and change email (#5956)

Author: marchersimon

contributing-guides/git-terminal.md

@@ -1,3 +1,5 @@
+# Opening a Pull Request
+
 Most people submit pull requests to the tldr-pages project
 [using GitHub's web interface][pr-howto].
 
@@ -29,3 +31,57 @@ Typically a pull request will include changes in a single file.
 [pr-howto]: ../CONTRIBUTING.md#submitting-a-pull-request
 [commit-msg]: ../CONTRIBUTING.md#commit-message
 [mass-changes]: https://github.com/tldr-pages/tldr/pulls?&q=is:pr+is:merged+label:"mass+changes"
+
+# Updating your fork
+
+Forks of GitHub repositories aren't updated automatically. To keep your fork up-to-date with the latest changes and avoid merge conflicts, you should update it regularly.
+
+There are two ways to update your fork.
+
+1. Via the GitHub web interface. Click `Fetch upstream` and then `Fetch and merge` on the fork as shown below:
+
+![Fetch and merge button in GitHub](../images/github-fetch-and-merge-button.png).
+
+2. Using Git in the terminal:
+
+```bash
+git checkout main
+git remote add upstream https://github.com/tldr-pages/tldr.git  # only run if you don't already have the upstream remote (check with "git remote -v")
+git fetch upstream main
+git rebase upstream/main     # in case you have any merge conflicts, click the link below to see how to resolve them
+git push --force-with-lease  # not needed if you only want to update your local repository
+```
+[How to resolve merge conficts](https://docs.github.com/en/github/collaborating-with-pull-requests/addressing-merge-conflicts/resolving-a-merge-conflict-using-the-command-line)
+
+# Changing the email of your last commit
+
+If the email that you used for the last commit isn't associated with your GitHub account, you can either add it [here](https://github.com/settings/emails) or change the email of the commit with the following commands:
+
+```bash
+git commit --amend --author="Your Name <[email protected]>"
+git push --force-with-lease
+```
+
+# Changing the email of any commit(s)
+
+Let's take this commit history as an example:
+
+| Commit Hash | Author Email
+|---|---
+| A | [email protected]
+| B | [email protected]
+| C | [email protected]
+| D | [email protected]
+| E | [email protected]
+| F (HEAD) | [email protected]
+
+To change the email of commts A and D, run
+
+```bash
+git reset A
+git commit --amend --author="Your Name <[email protected]>"
+git cherry-pick B^..D # re-apply commits B to D
+git commit --amend --author="Your Name <[email protected]>"
+git cherry-pick E^..HEAD
+git push --force-with-lease
+```