Fix sorting order of tags.

Before the fix, tags file sorting was done alphabetically.  That would
treat line numbers as strings, and so, for example, if the same tag was
placed on the same page on lines, say, 9 and 114, the order you would
get, 114 would go first, instead of 9.

Fix adds proper entries comparison to the sort function.

autoload/vimwiki/tags.vim

@@ -213,6 +213,38 @@ function! s:merge_tags(metadata, pagename, file_metadata) "{{{
   return metadata
 endfunction " }}}
 
+" s:tags_entry_cmp
+"   Compares two actual lines from tags file.  Return value is in strcmp style.
+"   See help on sort() -- that's what this function is going to be used for.
+"   See also s:write_tags_metadata below -- that's where we compose these tags
+"   file lines.
+"
+"   This function is needed for tags sorting, since plain sort() compares line
+"   numbers as strings, not integers, and so, for example, tag at line 14
+"   preceeds the same tag on the same page at line 9.  (Because string "14" is
+"   alphabetically 'less than' string "9".)
+function! s:tags_entry_cmp(i1, i2) "{{{
+  let items = []
+  for orig_item in [a:i1, a:i2]
+    let fields = split(orig_item, "\t")
+    let item = {}
+    let item.text = fields[0]."\t".fields[1]
+    let item.lineno = 0 + matchstr(fields[2], '\m\d\+')
+    call add(items, item)
+  endfor
+  if items[0].text > items[1].text
+    return 1
+  elseif items[0].text < items[1].text
+    return -1
+  elseif items[0].lineno > items[1].lineno
+    return 1
+  elseif items[0].lineno < items[1].lineno
+    return -1
+  else
+    return 0
+  endif
+endfunction " }}}
+
 " s:write_tags_metadata
 "   Saves metadata object into a file. Throws exceptions in case of problems.
 function! s:write_tags_metadata(metadata) "{{{
@@ -234,7 +266,7 @@ function! s:write_tags_metadata(metadata) "{{{
             \)
     endfor
   endfor
-  call sort(tags)
+  call sort(tags, "s:tags_entry_cmp")
   call insert(tags, "!_TAG_FILE_SORTED\t1\t")
   call writefile(tags, metadata_path)
 endfunction " }}}