git completions: make :/:-prefixed paths work everywhere on old git

git versions that only support porcelain v1 output (like on CentOS 7,
which has 1.8.3) weren't completing files prefixed with : correctly iff
the name after the colon was also a valid relative path.

Fixes the tests on CentOS 7.

share/completions/git.fish

@@ -487,6 +487,7 @@ function __fish_git_files
                 end
 
                 if test "$root/$relfile" = (pwd -P)/$relfile
+                    and not string match -q ':*' -- (commandline -ct)
                     set file $relfile
                 end