Git-svn remotes can have a prefix for their remotes. If I set a prefix
of 'svn/' then my remote trunk branch is called svn/trunk. Update the
script to use the svn-remote.*.fetch key to figure out how the 'trunk'
branch is mapped into the remotes namespace and apply this to the
current branch. This assumes branches are mapped into the same
namespace, which is likely. It also doesn't work for tags, but neither
did the old code.
With the switch bug fixed, __fish_git_prompt can be very slightly
simplified by not recording the exit status of every case in the
describe style switch individually.
__fish_git_prompt.fish is a complete port of the __git_ps1 function from
git-completion.bash, with the relevant configuration variables changed
and some extra configuration added (namely, control over individual
colors and the status indicator characters).
