Add command for close section for commit

[bakunyo]

This is an alternative PR #54

[jreybert]

Thanks bakunyo!

I merge it, but there is still something. You write well the ongoing commit message in COMMIT_EDITMSG, but I remember just now that I discard this file before opening the commit section wirth 'CC'. Then, what have been written in COMMIT_EDITMSG is lost.

I do not expect that the user use 'CA' after a 'CU' to retrieve the ongoing commit message. I do not want either a complex state machine in vimagit to discard or not the COMMIT_EDITMSG after a 'CC'.

I will investigate the precise COMMIT_EDITMSG behavior. My first lead will be to look at how git gui works (git gui save the current commit message). Anyway, if you still want to work on this, do not hesitate :)

[bakunyo]

@jreybert Thank you for merging this PR!
I have known the existence of COMMIT_EDITMSG, so I pleased.
I will send other PR if I get a change!