[StGIT RFC] Changing patch@branch syntax

The patch@branch syntax is annoying, at least for bash-completion
purposes: we don't want to provide all possible completions accross
all branches, yet we'd like to get completion for foreign patches.

Another place where I feel it is bad is when using the full
"patch@branch//top" syntax: the MSB is in the middle, and the LSB is
on the right.

Both issues would be solved by switching to a MSB ordering, with a way
to distinguish branchnames when given.  Something similar to pathnames
would fit well - eg. [/branchname/]patchname[//top].  However, I'm not
sure using slashes would be a good choice, precisely because of the
similarity with real pathnames.  But we don't have so many separator
chars that are not special in one way or another, and would require
quoting them to avoid more user confusion.

What do you think ?
-- 
Yann.