Fix filling in c-ts-mode (bug#72116)

The previous fix introduced a regression in the case when there's only a
single line in the block comment.  In that case we don't want to add a
start at the second line:

/* foo foo foo */ should => /* foo foo
                               foo */

rather than   /* foo foo
               * foo */

This commit fixes that.

* lisp/progmodes/c-ts-common.el:
(c-ts-common--fill-block-comment): Don't mask the /*.
(c-ts-common--adaptive-fill-prefix): New function.
(c-ts-common-comment-setup): Don't set adaptive-regexp, change
adaptive-fill-first-line-regexp to work with the new
adaptive-fill-function.
* test/lisp/progmodes/c-ts-mode-resources/filling.erts: New tests

1 files changed, 46 insertions, 0 deletions

diff --git a/test/lisp/progmodes/c-ts-mode-resources/filling.erts b/test/lisp/progmodes/c-ts-mode-resources/filling.erts
index e51e3658c83..e58b8e91c90 100644
--- a/test/lisp/progmodes/c-ts-mode-resources/filling.erts
+++ b/test/lisp/progmodes/c-ts-mode-resources/filling.erts
@@ -6,6 +6,52 @@ Code:
 
 Point-Char: |
 
+Name: Single line
+
+=-=
+/* woooomy woooomy woooomy woooomy woooomy woooomy woooomy woooomy woooomy */
+=-=
+/* woooomy woooomy woooomy woooomy woooomy woooomy woooomy woooomy
+   woooomy */
+=-=-=
+
+Name: Two lines
+
+=-=
+/* woooomy woooomy woooomy woooomy woooomy woooomy woooomy woooomy
+   woooomy woooomy woooomy woooomy woooomy woooomy woooomy woooomy woooomy */
+=-=
+/* woooomy woooomy woooomy woooomy woooomy woooomy woooomy woooomy
+   woooomy woooomy woooomy woooomy woooomy woooomy woooomy woooomy
+   woooomy */
+=-=-=
+
+Name: Two lines with star
+
+=-=
+/* woooomy woooomy woooomy woooomy woooomy woooomy woooomy woooomy
+ * woooomy woooomy woooomy woooomy woooomy woooomy woooomy woooomy woooomy
+ */
+=-=
+/* woooomy woooomy woooomy woooomy woooomy woooomy woooomy woooomy
+ * woooomy woooomy woooomy woooomy woooomy woooomy woooomy woooomy
+ * woooomy
+ */
+=-=-=
+
+Name: First line empty (bug#72116)
+
+=-=
+/**
+ * rsite nrsoti ernsto ierntires ntoritsen roitsenrsoit enrstoi ensrotie nrsit ensroit enrsoi ensrien
+ */
+=-=
+/**
+ * rsite nrsoti ernsto ierntires ntoritsen roitsenrsoit enrstoi
+ * ensrotie nrsit ensroit enrsoi ensrien
+ */
+=-=-=
+
 Name: Type 1
 
 =-=