patch 8.1.2379: using old C style comments

Problem:    Using old C style comments.
Solution:   Use // comments where appropriate.
diff --git a/src/filepath.c b/src/filepath.c
index 8da6e17..0fc1a5c 100644
--- a/src/filepath.c
+++ b/src/filepath.c
@@ -162,7 +162,7 @@
 	 * path with the remaining path at the tail.
 	 */
 
-	/* Compute the length of the new path. */
+	// Compute the length of the new path.
 	sfx_len = (int)(save_endp - endp) + 1;
 	new_len = len + sfx_len;
 
@@ -2367,7 +2367,7 @@
     r2 = mch_stat((char *)s2, &st2);
     if (r1 != 0 && r2 != 0)
     {
-	/* if mch_stat() doesn't work, may compare the names */
+	// if mch_stat() doesn't work, may compare the names
 	if (checkname)
 	{
 	    if (fnamecmp(exp1, s2) == 0)
@@ -3689,7 +3689,7 @@
     void
 addfile(
     garray_T	*gap,
-    char_u	*f,	/* filename */
+    char_u	*f,	// filename
     int		flags)
 {
     char_u	*p;