$ git init
Initialized empty Git repository in /home/markf/so/.git/
$ vi a.txt
$ git add .
$ git commit -m '1'
[master (root-commit) d92b23b] 1
 1 file changed, 1 insertion(+)
 create mode 100644 a.txt
$ git hist
* d92b23b 2015-08-29 | 1 (HEAD, master) [mark.fisher]

$ git checkout -b f1 master
Switched to a new branch 'f1'
$ vi a.txt
$ git add .
$ git commit -m '2 on f1'
[f1 1db4fed] 2 on f1
 1 file changed, 1 insertion(+), 1 deletion(-)

$ git checkout -b f2 master
Switched to a new branch 'f2'
$ git hist
* d92b23b 2015-08-29 | 1 (HEAD, master, f2) [mark.fisher]

$ vi a.txt
$ git add .
$ git commit -m '3 on f2'
[f2 807e4a7] 3 on f2
 1 file changed, 1 insertion(+), 1 deletion(-)

$ git hist
* 807e4a7 2015-08-29 | 3 on f2 (HEAD, f2) [mark.fisher]
* d92b23b 2015-08-29 | 1 (master) [mark.fisher]

$ git checkout master
Switched to branch 'master'

$ git merge --no-ff f2
Merge made by the 'recursive' strategy.
 a.txt | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

$ git hist
*   d193d0a 2015-08-29 | Merge branch 'f2' (HEAD, master) [mark.fisher]
|
| * 807e4a7 2015-08-29 | 3 on f2 (f2) [mark.fisher]
|/
* d92b23b 2015-08-29 | 1 [mark.fisher]

$ git merge --no-ff f1
Auto-merging a.txt
CONFLICT (content): Merge conflict in a.txt
Automatic merge failed; fix conflicts and then commit the result.

$ git status
On branch master
You have unmerged paths.
  (fix conflicts and run "git commit")

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

        both modified:   a.txt