Improve tree-sitter indent anchor prev-adaptive-prefix (bug#61314)

Now prev-adaptive-prefix looks at the current line and checks if it
begins with a prefix itself.  If it does, prev-adaptive-prefix tries
to place the anchor before the prefix on the previous line, rather
than after it.

 - prev line
 - this line -> This line starts with a "-", i.e., begins with a
                prefix, so we place the anchor at the beginning of the
                 "-" of the previous line, rather than after it

 - prev line
   this line -> This line doesn't start with a prefix, so the anchor
                is placed after the previous line's "-".

* doc/lispref/modes.texi (Parser-based Indentation): Update manual.
* lisp/treesit.el:
(treesit-simple-indent-presets): Add local variable
this-line-has-prefix, base what anchor to return on the value of
this-line-has-prefix and whether the prev line has a prefix.

doc/lispref/modes.texi

@@ -5111,8 +5111,11 @@ This anchor is a function that is called with 3 arguments: @var{node},
 @var{parent}, and @var{bol}.  It tries to go to the beginning of the
 previous non-empty line, and matches @code{adaptive-fill-regexp}.  If
 there is a match, this function returns the end of the match,
-otherwise it returns nil.  This anchor is useful for a
-@code{indent-relative}-like indent behavior for block comments.
+otherwise it returns nil.  However, if the current line begins with a
+prefix (e.g., ``-''), return the beginning of the prefix of the
+previous line instead, so that the two prefixes aligns.  This anchor
+is useful for a @code{indent-relative}-like indent behavior for block
+comments.
 
 @end ftable
 @end defvar

lisp/treesit.el

@@ -1186,12 +1186,18 @@ See `treesit-simple-indent-presets'.")
                   (skip-syntax-backward "-")
                   (point))))
         (cons 'prev-adaptive-prefix
-              (lambda (_n parent &rest _)
-                (let ((comment-start-bol
-                       (save-excursion
-                         (goto-char (treesit-node-start parent))
-                         (line-beginning-position))))
+              (lambda (_n parent bol &rest _)
+                (let (comment-start-bol
+                      this-line-has-prefix)
                   (save-excursion
+                    (goto-char (treesit-node-start parent))
+                    (setq comment-start-bol (line-beginning-position))
+
+                    (goto-char bol)
+                    (setq this-line-has-prefix
+                          (and (looking-at adaptive-fill-regexp)
+                               (match-string 1)))
+
                     (forward-line -1)
                     (and (>= (point) comment-start-bol)
                          adaptive-fill-regexp
@@ -1199,7 +1205,14 @@ See `treesit-simple-indent-presets'.")
                          ;; If previous line is an empty line, don't
                          ;; indent.
                          (not (looking-at (rx (* whitespace) eol)))
-                         (match-end 0))))))
+                         ;; Return the anchor.  If the indenting line
+                         ;; has a prefix and the previous line also
+                         ;; has a prefix, indent to the beginning of
+                         ;; prev line's prefix rather than the end of
+                         ;; prev line's prefix. (Bug#61314).
+                         (or (and this-line-has-prefix
+                                  (match-beginning 1))
+                             (match-end 0)))))))
         ;; TODO: Document.
         (cons 'grand-parent
               (lambda (_n parent &rest _)
@@ -1336,8 +1349,11 @@ prev-adaptive-prefix
 
     Goes to the beginning of previous non-empty line, and tries
     to match `adaptive-fill-regexp'.  If it matches, return the
-    end of the match, otherwise return nil.  This is useful for a
-    `indent-relative'-like indent behavior for block comments.")
+    end of the match, otherwise return nil.  However, if the
+    current line begins with a prefix, return the beginning of
+    the prefix of the previous line instead, so that the two
+    prefixes aligns.  This is useful for a `indent-relative'-like
+    indent behavior for block comments.")
 
 (defun treesit--simple-indent-eval (exp)
   "Evaluate EXP.

test/lisp/progmodes/c-ts-mode-resources/indent.erts

@@ -242,6 +242,21 @@ line 2
  */
 =-=-=
 
+Name: Block Comment prefixes (Bug#61314)
+
+=-=-=
+/*
+- item1
+- item2
+- item3
+*/
+=-=-=
+/*
+  - item1
+  - item2
+  - item3
+ */
+=-=-=
 
 Code:
   (lambda ()