Improve git prepending

Copy __fish_prepend_sudo.fish behaviour from fish master from 2020
november.

Install/Templates/config/fish/functions/git_write_keyword.fish

@@ -1,4 +1,12 @@
 function git_write_keyword --description 'Write git in terminal prompt'
-	commandline -C 0
-	commandline -i "git "
+    set -l cmd (commandline -po)
+    set -l cursor (commandline -C)
+    if test "$cmd[1]" != git
+        commandline -C 0
+        commandline -i "git "
+        commandline -C (math $cursor + 4)
+    else
+        commandline -r (string sub --start=5 (commandline -p))
+        commandline -C -- (math $cursor - 4)
+    end
 end