user-manual: listing commits reachable from some refs not others

This is just an amusing example raised by someone in irc.

Signed-off-by: "J. Bruce Fields" <bfields@citi.umich.edu>

Documentation/user-manual.txt

@@ -830,6 +830,54 @@ available
 Which shows that e05db0fd is reachable from itself, from v1.5.0-rc1, and
 from v1.5.0-rc2, but not from v1.5.0-rc0.
 
+[[showing-commits-unique-to-a-branch]]
+Showing commits unique to a given branch
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Suppose you would like to see all the commits reachable from the branch
+head named "master" but not from any other head in your repository.
+
+We can list all the heads in this repository with
+gitlink:git-show-ref[1]:
+
+-------------------------------------------------
+$ git show-ref --heads
+bf62196b5e363d73353a9dcf094c59595f3153b7 refs/heads/core-tutorial
+db768d5504c1bb46f63ee9d6e1772bd047e05bf9 refs/heads/maint
+a07157ac624b2524a059a3414e99f6f44bebc1e7 refs/heads/master
+24dbc180ea14dc1aebe09f14c8ecf32010690627 refs/heads/tutorial-2
+1e87486ae06626c2f31eaa63d26fc0fd646c8af2 refs/heads/tutorial-fixes
+-------------------------------------------------
+
+We can get just the branch-head names, and remove "master", with
+the help of the standard utilities cut and grep:
+
+-------------------------------------------------
+$ git show-ref --heads | cut -d' ' -f2 | grep -v '^refs/heads/master'
+refs/heads/core-tutorial
+refs/heads/maint
+refs/heads/tutorial-2
+refs/heads/tutorial-fixes
+-------------------------------------------------
+
+And then we can ask to see all the commits reachable from master
+but not from these other heads:
+
+-------------------------------------------------
+$ gitk master --not $( git show-ref --heads | cut -d' ' -f2 |
+				grep -v '^refs/heads/master' )
+-------------------------------------------------
+
+Obviously, endless variations are possible; for example, to see all
+commits reachable from some head but not from any tag in the repository:
+
+-------------------------------------------------
+$ gitk ($ git show-ref --heads ) --not  $( git show-ref --tags )
+-------------------------------------------------
+
+(See gitlink:git-rev-parse[1] for explanations of commit-selecting
+syntax such as `--not`.)
+
 [[making-a-release]]
 Creating a changelog and tarball for a software release
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~