vim-patch:8.2.1741: pathshorten() only supports using one character

Problem:    pathshorten() only supports using one character.
Solution:   Add an argument to control the length. (closes vim/vim#7006)
https://github.com/vim/vim/commit/6a33ef0deb5c75c003a9f3bd1c57f3ca5e77327e

Cherry-pick a line in test from patch 8.2.0634.
Use Nvim's config paths in docs.

shorten_dir() returning a pointer looks a bit confusing here, as it is
actually the same pointer passed to it, and it doesn't really reduce
much code, so change it back to void.

Assigning rettv->vval.v_string = NULL is not needed if a pointer is
within 64 bits. While this is usually the case, I'm not sure if it can
be taken for granted.

1 files changed, 15 insertions, 4 deletions

diff --git a/src/nvim/eval/funcs.c b/src/nvim/eval/funcs.c
index edf6ed3c12..475c6bfffb 100644
--- a/src/nvim/eval/funcs.c
+++ b/src/nvim/eval/funcs.c
@@ -6846,12 +6846,23 @@ static void f_or(typval_T *argvars, typval_T *rettv, FunPtr fptr)
  */
 static void f_pathshorten(typval_T *argvars, typval_T *rettv, FunPtr fptr)
 {
+  int trim_len = 1;
+
+  if (argvars[1].v_type != VAR_UNKNOWN) {
+    trim_len = (int)tv_get_number(&argvars[1]);
+    if (trim_len < 1) {
+      trim_len = 1;
+    }
+  }
+
   rettv->v_type = VAR_STRING;
-  const char *const s = tv_get_string_chk(&argvars[0]);
-  if (!s) {
-    return;
+  const char_u *p = (char_u *)tv_get_string_chk(&argvars[0]);
+  if (p == NULL) {
+    rettv->vval.v_string = NULL;
+  } else {
+    rettv->vval.v_string = vim_strsave(p);
+    shorten_dir_len(rettv->vval.v_string, trim_len);
   }
-  rettv->vval.v_string = shorten_dir((char_u *)xstrdup(s));
 }
 
 /*