completions/git: Terminate pathspec magic

Git's pathspec system is kind of annoying:

>  A pathspec that begins with a colon : has special meaning. In the short form, the leading colon : is followed by zero or more "magic signature" letters (which optionally is terminated by another colon :), and the remainder is the pattern to match against the path. The "magic signature" consists of ASCII symbols that are neither alphanumeric, glob, regex special characters nor colon. The optional colon that terminates the "magic signature" can be omitted if the pattern begins with a character that does not belong to "magic signature" symbol set and is not a colon.

So if we complete `:/foo`, that "works" because "f" is alphanumeric
and so the "/" is the only magic character here.

If, however the filename starts with a magic character, that's used as
a magic signature.

So we do what the docs say and terminate the magic signature after the
"/" (which means "from the repo root").

Fixes #9004

tests/checks/git.fish

@@ -40,6 +40,9 @@ touch foo
 complete -C'git add '
 #CHECK: foo	Untracked file
 
+complete -C'git add :'
+#CHECK: :/:foo	Untracked file
+
 git config alias.s status
 complete 'git s --s'
 # CHECK --short