rc diff: skip email quotes in diff-jump

When reading and writing emails that contain patches (possibly
email-quoted), it can be convenient to the jump to the source file.
Allow this by making diff-jump (bound to <ret> in git-diff
buffers) ignore leading email quotes ("> "). A line that starts with
"> " should not occur in a unified diff, so this won't affect other
use cases.

Observe that diff-jump even works around interleaved replies; they
will not affect the computed line numbers because we ignore lines
that don't match ^(> )*[ +-].

rc/filetype/diff.kak

@@ -32,7 +32,7 @@ Switches:
         set-register c %val{cursor_column}
         # If there is a "diff" line, we don't need to look further back.
         try %{
-            execute-keys %{<a-l><semicolon><a-?>^diff\b<ret><a-x>}
+            execute-keys %{<a-l><semicolon><a-?>^(?:> )*diff\b<ret><a-x>}
         } catch %{
             # A single file diff won't have a diff line. Start parsing from
             # the buffer start, so we can tell if +++/--- lines are headers
@@ -73,6 +73,7 @@ Switches:
                     $have_diff_line = 0;
                     $state = "header";
                     while (<STDIN>) {
+                        s/^(> )*//g;
                         $last_line = $_;
                         if (m{^diff\b}) {
                             $state = "header";