completions/git: improve idempotency in case of double load

As mentioned in the previous few commits and in #11535, running
"set fish_complete_path ..."  and "complete -C 'git ...'"  may result in
"share/completions/git.fish" being loaded multiple times.

This is usually fine because fish internally erases all cached completions
whenever fish_complete_path changes.

Unfortunately there is at least global variable that grows each time git.fish
is sourced. This doesn't make a functional difference but it does slow
down completions.  Fix that by resetting the variable at load time.

share/completions/git.fish

@@ -649,6 +649,7 @@ function __fish_git_aliased_command
     end
 end
 
+set -g __fish_git_aliases
 git config -z --get-regexp 'alias\..*' | while read -lz alias cmdline
     set -l command (__fish_git_aliased_command $cmdline)
     string match -q --regex '\w+' -- $command; or continue