Proposal for git stash : add --staged option

[edgar.hipp@netapsys.fr]

Hello,

There's some feature of git that I have been missing.
When you have a lot of unstaged files, and would like to test what 
happens if you undo some of the changes that you think are unecessary, 
you would rather keep a copy of those changes somewhere.

For example

Changed but not updated:
     M config_test.xml
     M config_real.xml

I have changed both config_test.xml and config_real.xml, but I think the 
changes made in config_test.xml are unnecessary. However, I would still 
like to keep them somewhere in case it breaks something.

In this case for example, I would like to be able to stash only the file 
config_test.xml

Eg:

git add config_test.xml
git stash --staged

So that after this, my git looks like this:

Changed but not updated:
     M config_real.xml

and my stash contains only the changes introduced in config_test.xml

`git stash --keep-index` doesn't give the necessary control, because it 
will still stash everything (and create unnecessary merge complications 
if I change the files and apply the stash)

Best,

Edgar