fix(zsh): Repair broken completion of --git-opt

The brace expansion for iterating over the `--git-opt` values from the
command line (i.e. `{2..2..$(($#git_opts - 2))}`) was broken in a couple
ways.

First, in `{n1..n2..n3}`, n2 is the end number and n3 is the step value,
but these were transposed.

Second, in order to avoid backwards sequences, we need to ensure that
the end number is greater than the start number. A condition is added to
test that.

Author: jpgrayson

completion/stgit.zsh

@@ -1226,21 +1226,25 @@ __stg_complete_git_opts() {
     # The option values are at the even indexes in the git_opts array.
     # The last option is skipped because it is the partially specified one being
     # completed.
-    for i in {2..2..$(($#git_opts - 2))}; do
-        # Need to strip any leading '=' because zparseopts will parse, for example,
-        # `--diff-opt=foo` as ('--diff-opt' '=foo').
-        words+=(${git_opts[i]#=})
-    done
+    if (( $#git_opts > 2 )); then
+        for i in {1..$(($#git_opts - 2))..2}; do
+            # Need to strip any leading '=' because zparseopts will parse, for example,
+            # `--diff-opt=foo` as ('--diff-opt' '=foo').
+            words+=(${git_opts[i+1]#=})
+        done
+    fi
 
     # If the user has not started typing the value, prime it with '-' to force
     # completing only the options (and not arguments) to the git command.
     : ${SUFFIX:-${PREFIX:=-}}
     words+=("$PREFIX$SUFFIX")
     (( CURRENT = $#words ))
 
-    for i in {2..2..$#git_opts_after}; do
-        words+=(${git_opts_after[i]#=})
-    done
+    if (( $#git_opts_after > 2 )); then
+        for i in {1..$(($#git_opts_after - 2))..2}; do
+            words+=(${git_opts_after[i+1]#=})
+        done
+    fi
 
     _message -e "git-$git_cmd-option" "git $git_cmd" &&
         _dispatch git git $commands[git]