Allow to edit the commit message

[Gianluca Recchia]

Hi,

I like how pass works overall and the way it integrates with Git is great!
However, there's one thing that I find slightly annoying: the default 
commit message is often not very descriptive of the change I made to an 
entry and I often find myself having to amend the commit in order to 
change the message.

I believe it would be an improvement in user experience if the user were 
given the ability to edit the commit message before committing, perhaps 
using the prepare-commit-msg hook to prefill the message buffer with 
what the default message would be, so that the user would only need to 
exit the editor if they're okay with the default message. Alternatively 
this could be optional in the local .gitconfig and overridable only for 
one command through a flag.

Best regards,
Gianluca