fix(hack): various fixes & improvements to cherry-pick script

[agilgur5]

Motivation

As Isitha and I now have Approver permissions and were working on the release rotation per #12592, we both used the script and both found some issues, one of which was mentioned in #11997 (comment)

Fix a few bugs in the script and make it more ergonomic

Modifications

bugfixes

• error out when the current branch that is checked out is not equivalent to the branch we're doing a cherry-pick on, as you can't cherry-pick onto other branches (see also this SO question)
• correctly order the commits in reverse chronological order -- the commits should be cherry-picked in the same order they were added
  • add a simple tac (reverse cat) to achieve this
• like the dry-run, commits that aren't going to be cherry-picked should be skipped
  • previously this would error out with no error message, which was very confusing

misc improvements

• enable dry-run by default, for safety and simplicity (the arg is now optional)

• add flags to git cherry-pick

  • use -Xpatience to run the slower patience diff strategy for hopefully less merge conflicts
  • use -x to add "(cherry picked from commit ...)"
    • this flag is very useful for posterity for backports as it creates a backlink to the original commit
• other improvements:

  • remove redundant set -eu -- this only needs to be set once in the script
  • rename variables and functions to be more explicit etc
    • e.g. use conventional get* functions
  • reduce nesting by removing the else

Verification

Run ./hack/cherry-pick.sh release-3.5 fix in a few variations

• on a different branch
• with and without dry run

[agilgur5]

For reference, since this script is not on the release-3.5 branch (or release-3.4 or earlier for that matter), I copied the script to /tmp, checked out the release branch, and then ran the /tmp script:

cp hack/cherry-pick.sh /tmp/cherry-pick.sh
git checkout release-3.5
. /tmp/cherry-pick.sh

[agilgur5]

Due to the above, I backported this to release-3.5 as c956d70