bug#44983: Truncate long lines of grep output

[Juri Linkov]

> Anyway, I found the shortest change needed to support ripgrep,
> and pushed to master.

Here is another patch needed to support rg because currently rg fails
when --color is used without a value.  OTOH, in grep --color is the same as
--color=auto, so this is a win-win situation:

diff --git a/lisp/progmodes/grep.el b/lisp/progmodes/grep.el
index 5dc99cc7e9..ef73dac4c0 100644
--- a/lisp/progmodes/grep.el
+++ b/lisp/progmodes/grep.el
@@ -79,7 +79,7 @@ grep-highlight-matches
 markers for highlighting and adds the --color option in front of
 any explicit grep options before starting the grep.
 
-When this option is `auto', grep uses `--color' to highlight
+When this option is `auto', grep uses `--color=auto' to highlight
 matches only when it outputs to a terminal (when `grep' is the last
 command in the pipe), thus avoiding the use of any potentially-harmful
 escape sequences when standard output goes to a file or pipe.
@@ -95,7 +95,7 @@ grep-highlight-matches
   :type '(choice (const :tag "Do not highlight matches with grep markers" nil)
                 (const :tag "Highlight matches with grep markers" t)
                 (const :tag "Use --color=always" always)
-                (const :tag "Use --color" auto)
+                (const :tag "Use --color=auto" auto)
                 (other :tag "Not Set" auto-detect))
   :set #'grep-apply-setting
   :version "22.1")
@@ -743,7 +743,7 @@ grep-compute-defaults
                                `(nil nil nil "--color" "x" ,(null-device))
                                nil 1)
                               (if (eq grep-highlight-matches 'always)
-                                  "--color=always" "--color"))
+                                  "--color=always" "--color=auto"))
                          "")
                         grep-options)))
        (unless grep-template
@@ -1000,7 +1000,7 @@ grep-expand-template
                             ((eq grep-highlight-matches 'always)
                              (push "--color=always" opts))
                             ((eq grep-highlight-matches 'auto)
-                             (push "--color" opts)))
+                             (push "--color=auto" opts)))
                            opts))
                 (excl . ,excl)
                 (dir . ,dir)