Merge null and without-null regexp alists (Bug#27840, Bug#27873)
* lisp/progmodes/grep.el (grep-mode-font-lock-keywords): Allow for NUL
characters following filename in grep context lines.
(grep--regexp-alist-column, grep--regexp-alist-bin-matcher)
(grep-with-null-regexp-alist, grep-fallback-regexp-alist): Remove.
(grep-regexp-alist): Recombine their contents here.
(grep-mode):
* lisp/cedet/semantic/symref/grep.el
(semantic-symref-parse-tool-output-one-line):
* lisp/progmodes/xref.el (xref-collect-matches): Use the variable
`grep-regexp-alist' rather than the function.