docs: Clean up Git guide article on working copies.
docs: Clean up Git guide article on working copies.

* repo => repository for more standard language.
* Delete 3 repeats of explaining the origin/upstream/local.
* Add some links.
* Update `git pull` language for rebase changes.

eeshangarg/zulip,kou/zulip,eeshangarg/zulip,zulip/zulip,punchagan/zulip,rht/zulip,hackerkid/zulip,rht/zulip,zulip/zulip,hackerkid/zulip,rht/zulip,andersk/zulip,rht/zulip,eeshangarg/zulip,punchagan/zulip,zulip/zulip,rht/zulip,zulip/zulip,eeshangarg/zulip,kou/zulip,hackerkid/zulip,andersk/zulip,hackerkid/zulip,hackerkid/zulip,andersk/zulip,zulip/zulip,eeshangarg/zulip,punchagan/zulip,punchagan/zulip,eeshangarg/zulip,kou/zulip,rht/zulip,andersk/zulip,eeshangarg/zulip,punchagan/zulip,kou/zulip,andersk/zulip,andersk/zulip,zulip/zulip,punchagan/zulip,kou/zulip,rht/zulip,zulip/zulip,kou/zulip,hackerkid/zulip,andersk/zulip,kou/zulip,punchagan/zulip,hackerkid/zulip