Add customizable to display raw bytes as hex

* src/xdisp.c (get_next_display_element): Dispatch used format string
for unprintables based on new display-raw-bytes-as-hex variable.
(display-raw-bytes-as-hex): New variable.  (Bug#27122)

* lisp/cus-start.el: Add defcustom form for display-raw-bytes-as-hex.

* doc/emacs/display.texi: Document the new variable.
* etc/NEWS: Mention display-raw-bytes-as-hex.

* test/manual/redisplay-testsuite.el (test-redisplay-5-toggle)
(test-redisplay-5): New tests.
(test-redisplay): Call test-redisplay-5.

1 files changed, 6 insertions, 0 deletions

diff --git a/doc/emacs/display.texi b/doc/emacs/display.texi
index a0d0792eacc..c4554eb3187 100644
--- a/doc/emacs/display.texi
+++ b/doc/emacs/display.texi
@@ -1763,3 +1763,9 @@ itself, in pixels; the default is 2.
 in text that is hard to read.  Call the function
 @code{tty-suppress-bold-inverse-default-colors} with a non-@code{nil}
 argument to suppress the effect of bold-face in this case.
+
+@vindex display-raw-bytes-as-hex
+  Raw bytes are displayed in octal format by default, for example a
+byte with a decimal value of 128 is displayed as @code{\200}.  To
+change display to the hexadecimal format of @code{\x80}, set the
+variable @code{display-raw-bytes-as-hex} to @code{t}.