Both basic and advanced merging operations.

git merge [-n] [--stat] [--no-commit] [--squash]
     [--[no-]edit]
     [-s <strategy>] [-X <strategy-option>] [-S[<keyid>]]
     [--[no-]allow-unrelated-histories]
     [--[no-]rerere-autoupdate] [-m <msg>]
     [-F <file>] [<commit>...]
git merge --abort
git merge --continue

If you have this situation:

              A---B---C topic
             /
        D---E---F---G master

followed by:

$ git checkout master
$ git merge topic

the end result is a new merge commit H:

              A---B---C topic
             /         \
        D---E---F---G---H master

If you have this:

              A---B---C topic
             /
        D---E master

followed by:

$ git checkout master
$ git merge topic

the default fast-forward merge will produce:

              A---B---C topic
             /          master
        D---E

From man git-branch:

With --contains, shows only the branches that contain the named
commit (in other words, the branches whose tip commits are
descendants of the named commit), --no-contains inverts it.
With --merged, only branches merged into the named commit (i.e.
the branches whose tip commits are reachable from the named
commit) will be listed. With --no-merged only branches not
merged into the named commit will be listed. If the <commit>
argument is missing it defaults to HEAD (i.e. the tip of the
current branch).

Examples:

$ git branch --merged master
$ git branch --no-merged
$ git branch --contains v4.19
$ git branch --no-contains v4.19

Imagine this change on the master branch:

$ git show master
commit 4a93eab592cf790838314b3799eb7a4faa213538 (HEAD -> master)
Author: Robert P. J. Day <rpjday@crashcourse.ca>
Date:   Fri Mar 1 08:17:50 2019 -0500

    Change to third edition

diff --git a/README.asc b/README.asc
index fa40bad..9fefbd5 100644
--- a/README.asc
+++ b/README.asc
@@ -1,4 +1,4 @@
-= Pro Git, Second Edition
+= Pro Git, Third Edition
 
 Welcome to the second edition of the Pro Git book.
 
$

And imagine this clearly-conflicting change on the branch 4th:

$ git show 4th
commit 9e679bc1c27d5d708d571ea1b47564f3a99df944 (4th)
Author: Robert P. J. Day <rpjday@crashcourse.ca>
Date:   Fri Mar 1 08:19:35 2019 -0500

    Fourth edition

diff --git a/README.asc b/README.asc
index fa40bad..1f0879f 100644
--- a/README.asc
+++ b/README.asc
@@ -1,4 +1,4 @@
-= Pro Git, Second Edition
+= Pro Git, Fourth Edition
 
 Welcome to the second edition of the Pro Git book.
 
$

Trying to merge will generate a merge conflict:

$ git checkout master
$ git merge 4th
Auto-merging README.asc
CONFLICT (content): Merge conflict in README.asc
Automatic merge failed; fix conflicts and then commit the result.
$

git status will tell you that you have a merge conflict;

$ git status
... snip ...
You have unmerged paths.
  (fix conflicts and run "git commit")
  (use "git merge --abort" to abort the merge)

Unmerged paths:
  (use "git add <file>..." to mark resolution)

	both modified:   README.asc

no changes added to commit (use "git add" and/or "git commit -a")

The merge conflict is identified in the README.asc file:

$ less README.asc
<<<<<<< HEAD
= Pro Git, Third Edition
=======
= Pro Git, Fourth Edition
>>>>>>> 4th

Welcome to the second edition of the Pro Git book.

... etc etc ...

So edit the file and resolve the conflict any way you want, such as:

$ less README.asc
= Pro Git, Fifth Edition
... snip ...

Stage the resolved file:

$ git add README.asc

You can tell the conflicts are now resolved:

$ git status
On branch master

All conflicts fixed but you are still merging.
  (use "git commit" to conclude merge)

Changes to be committed:

	modified:   README.asc

$

And commit:

$ git commit -m "Bumped merge up to fifth edition"

The final version of README.asc:

= Pro Git, Fifth Edition

Welcome to the second edition of the Pro Git book.
... etc ...

--ff
    When the merge resolves as a fast-forward, only update the
    branch pointer, without creating a merge commit. This is
    the default behavior.

--no-ff
    Create a merge commit even when the merge resolves as a
    fast-forward. This is the default behaviour when merging an
    annotated (and possibly signed) tag that is not stored in
    its natural place in refs/tags/ hierarchy.

--ff-only
    Refuse to merge and exit with a non-zero status unless the
    current HEAD is already up to date or the merge can be
    resolved as a fast-forward.