If merging that is really fast forwarding creates new commit [Was: Re: how to show log for only one branch]

[Liu Yubao <yubao.liu@gmail.com>]

Thanks to Junio for his patient explanation about branches in git, I find 
there is a subtle difference between GIT and regular VCS that can be easily
neglected by newbies.

I realize that git is a *content tracker*, it only creates commit object
when the corresponding tree is really modified, git records content merging
but not usual merging operation, that's why git is called a content tracker.
This explains why a merging that is really a fast forwarding doesn't create
any new commit.

This feature is different from many regular VCS like CVS and Subversion and
confuses newbies that come from them: mainline doesn't make sense too much,
'git log' shows many logs from other branches. In git, a branch is almost a
tag, you can't get the *track* of a branch(It's a pity reflog is only for
local purpose). I am used to one-trunk-and-more-side-branches way, every
branches are isolated clearly, git makes me very confused at the beginning.


Then, what bad *logical* problem will happen if a merging that is really a 
fast forwarding creates a new commit?

If we throw away all compatibility, efficiency, memory and disk consumption
problems,
(1) we can get the track of a branch without reflog because HEAD^1 is
always the tip of target branch(or working branch usually) before merging.

(2) with the track, branch mechanism in git is possibly easier to understand,
especially for newbies from CVS or Subversion, I really like git's light 
weight, simple but powerful design and great efficiency, but I am really
surprised that 'git log' shows logs from other branches and a side branch can 
become part of main line suddenly.

A revision graph represents fast forwarding style merging like this:

             (fast forwarding)
  ---- a ............ * ------> master
        \            /
         b----------c -----> test         (three commits with three trees)

can be changed to:

  ---- a (tree_1) ----------- d (tree_3) ------> master
        \                    /
         b (tree_2) ------- c (tree_3) ----> test
(four commits with three trees, it's normal as more than one way can reach 
Rome :-)


I don't think I am smarter than any people in this mailing list, in fact
I am confused very much by GIT's branches at the beginning. There must
be many problems I haven't realized, I am very curious about them, any