This is a bad idea masquerading as a good idea. Before making a pull request (or doing any sort of merge), you should rebase against upstream master (or whatever you're going to push to). However, keeping distinct atomic commits that change one and only one small thing, when possible, is much preferable if bisect or blame is used. If you have broken or poorly written commits, use fixup, reword, squash, etc. in rebase -i.
Using fast-forward (and possibly only allowing fast-forward) is a good idea. Squashing entire pull requests that may change multiple things into a single commit is a very bad idea.
Using fast-forward (and possibly only allowing fast-forward) is a good idea. Squashing entire pull requests that may change multiple things into a single commit is a very bad idea.