patch 8.2.4907: some users do not want a line comment always inserted

Problem:    Some users do not want a line comment always inserted.
Solution:   Add the '/' flag to 'formatoptions' to not repeat the comment
            leader after a statement when using "o".

runtime/doc/change.txt

@@ -1673,9 +1673,9 @@ readability.
 
 letter	 meaning when present in 'formatoptions'    ~
 							*fo-t*
-t	Auto-wrap text using textwidth
+t	Auto-wrap text using 'textwidth'
 							*fo-c*
-c	Auto-wrap comments using textwidth, inserting the current comment
+c	Auto-wrap comments using 'textwidth', inserting the current comment
 	leader automatically.
 							*fo-r*
 r	Automatically insert the current comment leader after hitting
@@ -1684,6 +1684,9 @@ r	Automatically insert the current comment leader after hitting
 o	Automatically insert the current comment leader after hitting 'o' or
 	'O' in Normal mode.  In case comment is unwanted in a specific place
 	use CTRL-U to quickly delete it. |i_CTRL-U|
+							*fo-/*
+/	When 'o' is included: do not insert the comment leader for a //
+	comment after a statement, only when // is at the start of the line.
 							*fo-q*
 q	Allow formatting of comments with "gq".
 	Note that formatting will not change blank lines or lines containing
@@ -1746,8 +1749,8 @@ B	When joining lines, don't insert a space between two multibyte
 1	Don't break a line after a one-letter word.  It's broken before it
 	instead (if possible).
 							*fo-]*
-]	Respect textwidth rigorously. With this flag set, no line can be
-	longer than textwidth, unless line-break-prohibition rules make this
+]	Respect 'textwidth' rigorously. With this flag set, no line can be
+	longer than 'textwidth', unless line-break-prohibition rules make this
 	impossible.  Mainly for CJK scripts and works only if 'encoding' is
 	"utf-8".
 							*fo-j*

src/change.c

@@ -1659,7 +1659,8 @@ open_line(
 	lead_len = get_leader_len(saved_line, &lead_flags,
 							dir == BACKWARD, TRUE);
 #ifdef FEAT_CINDENT
-	if (lead_len == 0 && curbuf->b_p_cin && do_cindent && dir == FORWARD)
+	if (lead_len == 0 && curbuf->b_p_cin && do_cindent && dir == FORWARD
+					&& !has_format_option(FO_NO_OPEN_COMS))
 	{
 	    // Check for a line comment after code.
 	    comment_start = check_linecomment(saved_line);

src/option.h

@@ -141,6 +141,7 @@ typedef enum {
 #define FO_WRAP_COMS	'c'
 #define FO_RET_COMS	'r'
 #define FO_OPEN_COMS	'o'
+#define FO_NO_OPEN_COMS	'/'
 #define FO_Q_COMS	'q'
 #define FO_Q_NUMBER	'n'
 #define FO_Q_SECOND	'2'
@@ -159,7 +160,7 @@ typedef enum {
 
 #define DFLT_FO_VI	"vt"
 #define DFLT_FO_VIM	"tcq"
-#define FO_ALL		"tcroq2vlb1mMBn,aw]jp"	// for do_set()
+#define FO_ALL		"tcro/q2vlb1mMBn,aw]jp"	// for do_set()
 
 // characters for the p_cpo option:
 #define CPO_ALTREAD	'a'	// ":read" sets alternate file name
@@ -196,7 +197,7 @@ typedef enum {
 #define CPO_REMMARK	'R'	// remove marks when filtering
 #define CPO_BUFOPT	's'
 #define CPO_BUFOPTGLOB	'S'
-#define CPO_TAGPAT	't'
+#define CPO_TAGPAT	't'	// tag pattern is used for "n"
 #define CPO_UNDO	'u'	// "u" undoes itself
 #define CPO_BACKSPACE	'v'	// "v" keep deleted text
 #define CPO_CW		'w'	// "cw" only changes one blank

src/testdir/test_textformat.vim

@@ -278,6 +278,18 @@ func Test_format_c_comment()
                       //
   END
   call assert_equal(expected, getline(1, '$'))
+  3delete
+
+  " No comment repeated with a slash in 'formatoptions'
+  set fo+=/
+  normal 2Gox
+  let expected =<< trim END
+      nop;
+      val = val;      // This is a comment
+      x
+  END
+  call assert_equal(expected, getline(1, '$'))
+  set fo-=/
 
   " using 'indentexpr' instead of 'cindent' does not repeat a comment
   setl nocindent indentexpr=2

src/version.c

@@ -746,6 +746,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4907,
 /**/
     4906,
 /**/