vim-patch:8.2.0865 syntax: Add command to control how foldlevel is computed

Problem:    Syntax foldlevel is taken from the start of the line.
Solution:   Add ":syn foldlevel" to be able to use the minimal foldlevel in the line.
e35a52aee7

With `foldmethod=syntax` the foldlevel of a line is computed based
on syntax items on the line.  Previously we always used the level
of the syntax item containing the start of the line.  This works
well in cases such as:

    if (...) {
      ...
    }
    else if (...) {
      ...
    }
    else {
      ...
    }

which folds like this:

    +---  3 lines: if (...) {---------------------------
    +---  3 lines: else if (...) {----------------------
    +---  3 lines: else {-------------------------------

However, the code:

    if (...) {
      ...
    } else if (...) {
      ...
    } else {
      ...
    }

folds like this:

    +---  7 lines: if (...) {---------------------------

We can make the latter case fold like this:

    +---  2 lines: if (...) {---------------------------
    +---  2 lines: } else if (...) {--------------------
    +---  3 lines: } else {-----------------------------

by choosing on each line the lowest fold level that is followed
by a higher fold level.

Add a syntax command

    :syntax foldlevel [start | minimum]

to choose between these two methods of computing the foldlevel of
a line.

src/nvim/syntax.c

@@ -3079,6 +3079,41 @@ static void syn_cmd_case(exarg_T *eap, int syncing)
   }
 }
 
+/// Handle ":syntax foldlevel" command.
+static void syn_cmd_foldlevel(exarg_T *eap, int syncing)
+{
+  char_u *arg = eap->arg;
+  char_u *arg_end;
+
+  eap->nextcmd = find_nextcmd(arg);
+  if (eap->skip)
+    return;
+
+  if (*arg == NUL) {
+    switch (curwin->w_s->b_syn_foldlevel) {
+    case SYNFLD_START:   MSG(_("syntax foldlevel start"));   break;
+    case SYNFLD_MINIMUM: MSG(_("syntax foldlevel minimum")); break;
+    default: break;
+    }
+    return;
+  }
+
+  arg_end = skiptowhite(arg);
+  if (STRNICMP(arg, "start", 5) == 0 && arg_end - arg == 5) {
+    curwin->w_s->b_syn_foldlevel = SYNFLD_START;
+  } else if (STRNICMP(arg, "minimum", 7) == 0 && arg_end - arg == 7) {
+    curwin->w_s->b_syn_foldlevel = SYNFLD_MINIMUM;
+  } else {
+    EMSG2(_(e_illegal_arg), arg);
+    return;
+  }
+
+  arg = skipwhite(arg_end);
+  if (*arg != NUL) {
+    EMSG2(_(e_illegal_arg), arg);
+  }
+}
+
 /*
  * Handle ":syntax spell" command.
  */
@@ -3163,6 +3198,7 @@ void syntax_clear(synblock_T *block)
   block->b_syn_error = false;           // clear previous error
   block->b_syn_slow = false;            // clear previous timeout
   block->b_syn_ic = false;              // Use case, by default
+  block->b_syn_foldlevel = SYNFLD_START;
   block->b_syn_spell = SYNSPL_DEFAULT;  // default spell checking
   block->b_syn_containedin = false;
   block->b_syn_conceal = false;
@@ -5487,6 +5523,7 @@ static struct subcommand subcommands[] =
   { "cluster",   syn_cmd_cluster },
   { "conceal",   syn_cmd_conceal },
   { "enable",    syn_cmd_enable },
+  { "foldlevel", syn_cmd_foldlevel },
   { "include",   syn_cmd_include },
   { "iskeyword", syn_cmd_iskeyword },
   { "keyword",   syn_cmd_keyword },
@@ -5789,7 +5826,24 @@ int syn_get_foldlevel(win_T *wp, long lnum)
       && !wp->w_s->b_syn_slow) {
     syntax_start(wp, lnum);
 
+    // Start with the fold level at the start of the line.
     level = syn_cur_foldlevel();
+
+    if (wp->w_s->b_syn_foldlevel == SYNFLD_MINIMUM) {
+      // Find the lowest fold level that is followed by a higher one.
+      int cur_level = level;
+      int low_level = cur_level;
+      while (!current_finished) {
+        (void)syn_current_attr(false, false, NULL, false);
+        cur_level = syn_cur_foldlevel();
+        if (cur_level < low_level) {
+          low_level = cur_level;
+        } else if (cur_level > low_level) {
+          level = low_level;
+        }
+        current_col++;
+      }
+    }
   }
   if (level > wp->w_p_fdn) {
     level = wp->w_p_fdn;