Search for already present tag links

If the tag link section should only be updated, search for an already
existing section and use the present tags for updating the links.

autoload/vimwiki/tags.vim

@@ -325,8 +325,33 @@ function! vimwiki#tags#generate_tags(create, ...) abort
   " Similar to vimwiki#base#generate_links.  In the current buffer, appends
   " tags and references to all their instances.  If no arguments (tags) are
   " specified, outputs all tags.
-  let specific_tags = a:000
   let header_level = vimwiki#vars#get_global('tags_header_level')
+  let tags_header_text = vimwiki#vars#get_global('tags_header')
+
+  " If tag headers should only be updated, search for already present tag headers
+  if !a:create
+    let headers = vimwiki#base#collect_headers()
+    let specific_tags = []
+    let inside_tag_headers = 0
+    for header in headers
+      " If we ran out of the headers containing the tags, stop
+      if inside_tag_headers && header[1] <= header_level
+        break
+      endif
+
+      " All headers in the tag headers section correspond to tag names. Collect all of them in a list.
+      if inside_tag_headers
+        call add(specific_tags, header[2])
+      endif
+
+      " If we found the start of the tag headers section, remember that the following headers are now inside of it
+      if header[1] == header_level && header[2] ==# tags_header_text
+        let inside_tag_headers = 1
+      endif
+    endfor
+  else
+    let specific_tags = a:000
+  endif
 
   " use a dictionary function for closure like capability
   " copy all local variables into dict (add a: if arguments are needed)
@@ -407,7 +432,7 @@ function! vimwiki#tags#generate_tags(create, ...) abort
 
   call vimwiki#base#update_listing_in_buffer(
         \ GeneratorTags,
-        \ vimwiki#vars#get_global('tags_header'),
+        \ tags_header_text,
         \ links_rx,
         \ line('$')+1,
         \ header_level,