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 #7006)
diff --git a/src/proto/filepath.pro b/src/proto/filepath.pro
index b0a6de9..b502b8e 100644
--- a/src/proto/filepath.pro
+++ b/src/proto/filepath.pro
@@ -21,6 +21,7 @@
 void f_globpath(typval_T *argvars, typval_T *rettv);
 void f_isdirectory(typval_T *argvars, typval_T *rettv);
 void f_mkdir(typval_T *argvars, typval_T *rettv);
+void shorten_dir(char_u *str);
 void f_pathshorten(typval_T *argvars, typval_T *rettv);
 void f_readdir(typval_T *argvars, typval_T *rettv);
 void f_readdirex(typval_T *argvars, typval_T *rettv);
@@ -40,7 +41,6 @@
 char_u *get_past_head(char_u *path);
 int vim_ispathsep(int c);
 int vim_ispathsep_nocolon(int c);
-void shorten_dir(char_u *str);
 int dir_of_file_exists(char_u *fname);
 int vim_fnamecmp(char_u *x, char_u *y);
 int vim_fnamencmp(char_u *x, char_u *y, size_t len);