Disable svn prompt by default

This is apparently quite slow on large svn repos (like 40 seconds
slow), and we don't have a good thing to display other than the full
file information.

So we'll have to disable it for now.

Fixes #6681.

[ci skip]

doc_src/cmds/fish_vcs_prompt.rst

@@ -25,6 +25,8 @@ It calls out to VCS-specific functions. The currently supported systems are:
 
 If a VCS isn't installed, the respective function does nothing.
 
+The svn prompt is disabled by default because it's slow on large svn repositories. To enable it, modify fish_vcs_prompt to uncomment it. See :ref:`funced <cmd-funced>`.
+
 For more information, see the documentation for each of the functions above.
 
 Example

share/functions/fish_vcs_prompt.fish

@@ -3,5 +3,8 @@ function fish_vcs_prompt --description "Print the prompts for all available vcse
     # This is so we don't try svn if git already worked.
     fish_git_prompt
     or fish_hg_prompt
-    or fish_svn_prompt
+    # The svn prompt is disabled by default because it's quite slow on common svn repositories.
+    # To enable it uncomment it.
+    # You can also only use it in specific directories by checking $PWD.
+    # or fish_svn_prompt
 end