[Search git status] rewrite parsing logic without regex and fix bug w…

…ith renames

There was a bug in the status parsing logic in which the unstaged output header would be printed if a file was renamed in the index but had no unstaged changes. I decided the regex code was too hard to read and unwieldy--hence how the bug survived code review--so rewrote it using simple, self-documenting substring checks while fixing the bug. This should also marginally improve performance.

functions/_fzf_preview_changed_file.fish

@@ -3,17 +3,23 @@
 # MM functions/_fzf_preview_changed_file.fish
 function _fzf_preview_changed_file
     set -l path (string split ' ' $argv)[-1]
-    if string match -r '^\?\?' $argv --quiet
+    # first letter of short format shows index, second letter shows working tree
+    # https://git-scm.com/docs/git-status/2.35.0#_output
+    set -l index_status (string sub --length 1 $argv)
+    set -l working_tree_status (string sub --start 2 --length 1 $argv)
+
+    if test $index_status = '?'
         echo -e (set_color --underline)=== Untracked ===\n
         _fzf_preview_file $path
     else
-        if string match -r '\S\s\S' $argv --quiet
-            echo -e (set_color --underline red)=== Unstaged ===\n
-            git diff --color=always -- $path
-        end
-        if string match -r '^\S' $argv --quiet
+        if test $index_status != ' '
             echo -e (set_color --underline green)=== Staged ===\n
             git diff --color=always --staged -- $path
         end
+
+        if test $working_tree_status != ' '
+            echo -e (set_color --underline red)=== Unstaged ===\n
+            git diff --color=always -- $path
+        end
     end
 end