What you're trying to do, I think, is something I've struggled with a few years back too, and I've come to the following solution and conclusions and processes:
There is no sense in uncommitting my_a with git reset HEAD^ or git reset HEAD~ just so you can then git difftool it immediately to see what changes my_a is going to make.
Rather, just check those changes withOUT doing git reset HEAD~, like this:
# Run after the rebase
git diff HEAD~..HEAD
Even better, set meld as your difftool and do:
git difftool HEAD~..HEAD
Then, if you see anything wrong, fix it and make a new commit with commit message Fix errors from rebase:
# run after the rebase
git difftool HEAD~..HEAD
# If you see any errors, fix them, then commit everything and make a new commit
git add -A
git commit -m "Fix errors from rebase"
But, even better than that, in my opinion, is to see what changes were added upstream in develop that you do NOT have in my_a. You need to check these additions you did NOT do to ensure they do NOT conflict with your changes!
To do that, run this after the rebase:
git difftool my_a_BEFORE_THE_REBASE my_a
But, to do that you need to know the commit hash before you rebased, so, use this workflow instead in order to create a backup branch to compare against:
# before the rebase:
# ensure you are on your feature branch, my_a
git checkout my_a
# make a backup branch of branch my_a
git branch my_a_BAK
# Fetch latest develop withOUT having to check it out first
git fetch origin develop:develop
# rebase onto latest develop
git rebase develop
# manually fix any conflicts and then continue
git rebase --continue
# after the rebase:
# compare against your backup branch to look at upstream changes added into
# develop to ensure they do NOT conflict with your changes
git difftool my_a_BAK my_a
# If you see any errors, fix them, then commit everything and make a new commit
git add -A
git commit -m "Fix errors from rebase"
BUT, there's a big problem! What if develop is a HUGE repo used by hundreds of developers who just added HUNDREDS of new files!?
Now, git difftool my_a_BAK my_a will show you all of these hundreds of new files that have nothing to do with your changes!
Oh, wo is me! Wo is me! :)
The solution is to only check the files you touched by my_a. To do that, you need to run this:
# NOT this; it could show HUNDREDS of files!
git difftool my_a_BAK my_a
# do this instead
git difftool my_a_BAK my_a -- myfile1.c myfile2.c myfile3.c
...where myfile1.c myfile2.c myfile3.c are the only files you touched.
To see a list of all files you touched, run this (yes, with 3 dots):
git diff --name-only develop...my_a
Then, manually type those files into the command above in place of myfile1.c myfile2.c myfile3.c.
Well shoot! That's a pain! What if I have 40 files? I don't want to copy-paste all that garbage!
Final answer
That's why I wrote git-changes.sh. It solves all those issues above, and gets all those files for you.
Run it in your workflow like this:
# before the rebase:
# ensure you are on your feature branch, my_a
git checkout my_a
# make a backup branch of branch my_a
git branch my_a_BAK
# Fetch latest develop withOUT having to check it out first
git fetch origin develop:develop
# rebase onto latest develop
git rebase develop
# manually fix any conflicts, `git add` the files, and then continue with:
git rebase --continue
# after the rebase:
# Compare against your backup branch to look at upstream changes added into
# develop to ensure they do NOT conflict with your changes.
# The general format of `git changes` is
# `git changes <common_base> <backup_branch>`. Run `git changes` with
# NO args to see the help menu.
git changes develop my_a_BAK # <=== my script ===
# If you see any errors, fix them, then commit everything and make a new commit
git add -A
git commit -m "Fix errors from rebase"
To get git changes as a command, just rename git-changes.sh to git-changes and move it to any directory already in your PATH, such as to the ~/bin dir if using Ubuntu with a default ~/.profile file. Note: if you don't have a default ~/.profile file, get it from /etc/skel/.profile:
# copy the default .profile file to your home dir in Ubuntu
cp -i /etc/skel/.profile ~
Run git changes with no arguments to see its full help menu.
