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)
6a33ef0deb

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.

src/nvim/path.c

@@ -269,16 +269,17 @@ int vim_ispathlistsep(int c)
 #endif
 }
 
-/*
- * Shorten the path of a file from "~/foo/../.bar/fname" to "~/f/../.b/fname"
- * It's done in-place.
- */
-char_u *shorten_dir(char_u *str)
+/// Shorten the path of a file from "~/foo/../.bar/fname" to "~/f/../.b/fname"
+/// "trim_len" specifies how many characters to keep for each directory.
+/// Must be 1 or more.
+/// It's done in-place.
+void shorten_dir_len(char_u *str, int trim_len)
 {
   char_u *tail = path_tail(str);
   char_u *d = str;
   bool skip = false;
-  for (char_u *s = str;; ++s) {
+  int dirchunk_len = 0;
+  for (char_u *s = str;; s++) {
     if (s >= tail) {                // copy the whole tail
       *d++ = *s;
       if (*s == NUL) {
@@ -287,10 +288,16 @@ char_u *shorten_dir(char_u *str)
     } else if (vim_ispathsep(*s)) {       // copy '/' and next char
       *d++ = *s;
       skip = false;
+      dirchunk_len = 0;
     } else if (!skip) {
       *d++ = *s;                     // copy next char
       if (*s != '~' && *s != '.') {  // and leading "~" and "."
-        skip = true;
+        dirchunk_len++;  // only count word chars for the size
+        // keep copying chars until we have our preferred length (or
+        // until the above if/else branches move us along)
+        if (dirchunk_len >= trim_len) {
+          skip = true;
+        }
       }
       int l = utfc_ptr2len(s);
       while (--l > 0) {
@@ -298,7 +305,13 @@ char_u *shorten_dir(char_u *str)
       }
     }
   }
-  return str;
+}
+
+/// Shorten the path of a file from "~/foo/../.bar/fname" to "~/f/../.b/fname"
+/// It's done in-place.
+void shorten_dir(char_u *str)
+{
+  shorten_dir_len(str, 1);
 }
 
 /*