Suppose you’ve been working in branch X and you’re about to merge those changes into branch Y …

• work in branch X and make your commits as needed
• switch to branch Y git checkout Y
• compare the differences between X and Y git log Y..X – the order here is important; it is merge-target..merge-source
• merge the changes from X (merge-source) into Y (merge-target) git merge X

Even better, before doing the merge, while you are still in branch X, you can squash multiple commits into a single one. This way, branch Y, when you examine the log, has a single entry “implemented feature X”, instead of thirty-five entries all related to feature X in some way.

• examine the log git log or even better git log --pretty=oneline
• choose the commit that immediately precedes the one that starts your piece of work (probably the last one before you branched to start work on feature X)
• start an interactive rebase session – this is git-fancy-talk for picking some commits and typing a message – git rebase -i THE-COMMIT-ID-OF-THE-COMMIT-YOU-SELECTED-ABOVE
• Your favourite editor (or nano) will open listing the commits for feature X – edit the word “pick” to “squash” for all except the top entry, save and exit
• Your favourite editor will open again – again showing your commit messages – add a line at the top stating something like “implementation of feature X” and leave the list of individual commits on separate lines below it (adding in a Fixes #123 or whatever your issue tracker demands as the last line), save and exit
• Now if you do a git log you will see a single commit with a nice “implementation of feature X” log message
• And you’re all set to merge to your master branch without cluttering its history with a long string of commit messages