git,commit,branch ,cherry-pick ,merge

• commit 
• is snapshot of all files
• not a change set
•  to store the hash ID of the latest commit in a branch name.
• branch name is just the lastest hash of commi branch name is just the lastest hash of commit
• In fact, this is the very definition of a branch name: a name like master simply stores the hash ID of the commit we want to call the latest for that branch. So given the A--B--C string of commits, we just add the name master, pointing to commit C:

• To remember which branch we want Git to update, as well as which commit we have checked-out right now, Git has the special name HEAD
• current branch
• A--B--C--D <-- develop (HEAD), master
• HEAD still names the current commit.
• A detached HEAD just means that Git has made the name HEAD point directly to some commit instead of attaching it to a branch name

• git merge
• git checkout master
• set HEAD to master
• git merge develop
• go backwards from master and develop, to find a shared base commit (祖先), called merge base
• The merge base commit is determined entirely by the commit graph,
• git diff baseCommint master  => get change set 1 
• git diff basecommit  develop   => get change set 2
• git combine the difference , use  a new commit id to save the result  ( merge commit )
• cherry-pick
• git diff somecommitId to get the change set
• just put the change on current commit(branch)
• but it maybe not works very well, git use cherry-pick-style merge, force the merge base commit to be the parent of the cherry-picked commit.
• cherry-pick create a new commit to save the result. 

•        G--H--F'   <-- master (HEAD)
        /
  ...--D
        \
         E--F   <-- develop

•  cherry-pick uses Git's merge machinery to do the change-combining, but leaves the graph unmerged, simply making a copy of some existing commit

• git merge somecommitid(branch)
• git merge -no-ff