emacs: also handle paths like "..\chrome/foo/bar" seen in build output

tools/emacs/trybot.el

 ; To use this,
 ; 1) Add to init.el:
 ;    (setq-default chrome-root "/path/to/chrome/src/")
 ;    (add-to-list 'load-path (concat chrome-root "tools/emacs"))
 ;    (require 'trybot)
 ; 2) Run on trybot output:
 ;    M-x trybot
 ;
 ; To hack on this,
 ; M-x eval-buffer
@@ skipping 27 matching lines @@
       (if corrected-path
           (file-relative-name (directory-file-name corrected-path)
                               (get-chrome-root))
         nil))))
 
 (defun trybot-fixup-win ()
   "Fix up Windows-specific output."
 
   ; Fix Windows paths ("d:\...\src\").
   (save-excursion
-    (while (re-search-forward "\\(^.:\\\\.*\\\\src\\\\\\)\\(.*?\\)[(:]" nil t)
+    ; This regexp is subtle and rather hard to read. :~(
+    ; Use regexp-builder when making changes to it.
+    (while (re-search-forward
+            (concat
+             ; First part: path leader, either of the form
+             ;   e:\...src\  or  ..\
+             "\\(^.:\\\\.*\\\\src\\\\\\|\\.\\.\\\\\\)"
+             ; Second part: path, followed by error message marker.
+             "\\(.*?\\)[(:]") nil t)
       (replace-match "" nil t nil 1)
       ; Line now looks like:
       ;  foo\bar\baz.cc error message here
       ; We want to fixup backslashes in path into forward slashes,
       ; without modifying the error message - by matching up to the
       ; first colon above (which will be just beyond the end of the
       ; filename) we can use the end of the match as a limit.
       (subst-char-in-region (point) (match-end 0) ?\\ ?/)
       ; See if we can correct the file name casing.
       (let ((filename (buffer-substring (match-beginning 2) (match-end 2))))
@@ skipping 100 matching lines @@
     (setq url (concat url "/text")))
 
   (let ((type-hint (cond ((string-match "/[Ww]in" url) 'win)
                          ((string-match "/mac/" url) 'mac)
                          ; Match /linux, /linux_view, etc.
                          ((string-match "/linux" url) 'linux)
                          (t 'unknown))))
     (trybot-fetch type-hint url)))
 
 (provide 'trybot)