Avoid SIGPIPE (error 141) in auto-cancel Run Step example

[davidjb]

The example shown at https://support.circleci.com/hc/en-us/articles/360055901372-Auto-Cancel-Re-Run-if-Newer-Commit-Exists is susceptible to SIGPIPE (Bash exit code 141) errors because of the mixture of commands. In short, use of head -n 1 exits before the git show command ends, because git show is still busy outputting the diff of a given commit. This also created a race condition where shorter diffs would not trigger the issue but longer ones would. This is solved with the changes in this PR, by not outputting the patches.

This also cleans up the first "latest commit" command by simplifying it to avoid needing grep. By doing so, it focussing on --heads rather than potentially tags as well and fixes a situation where grep would match multiple branches -- e.g. dev matches another-dev, leading to incorrect tests.