bug#59575: 29.0.50; add-log-current-defun-header-regexp matches Windows drive letter

[Juanma Barranquero <lekktu@gmail.com>]

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If you have an *xref* buffer with absolute Windows filenames, like

~/.emacs.d/init.el
  93:             server-name    (replace-regexp-in-string "\\\\+" "\\"
serv t t)
1102:   (let ((s (when server-name
1104:                (string-match (rx (+ (not (any ?\\))) line-end)
server-name)
1105:                (upcase (match-string 0 server-name))))))
d:/Devel/emacs/repo/trunk/lisp/erc/erc-backend.el
1820:   (pcase-let ((`(,server-name ,server-version)
1823:     (setq erc-server-announced-name server-name)
1827:      's004 ?s server-name ?v server-version

and put the cursor in an absolute filename line (like the one
d:/Devel/[etc] above), calling `add-log-current-defun' returns the drive
letter "d", because it matches a-l-c-d-header-regexp.

The effect is visible when you have which-function-mode enabled, because
the function returns non-nil, so which-function does not resort to imenu,
and you end with "d" in the mode-line.

I suppose this should be fixed in xref.el, which apparently assumes that
file name lines will be either relative or Unix-style (/path/file works
correctly, it's just d:/path/file that fails) and the add-log heuristics
will always fail.

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