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/evalfunc.c b/src/evalfunc.c
index 9bdeb52..57c4ffb 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -779,7 +779,7 @@
     {"nextnonblank",	1, 1, FEARG_1,	  ret_number,	f_nextnonblank},
     {"nr2char",		1, 2, FEARG_1,	  ret_string,	f_nr2char},
     {"or",		2, 2, FEARG_1,	  ret_number,	f_or},
-    {"pathshorten",	1, 1, FEARG_1,	  ret_string,	f_pathshorten},
+    {"pathshorten",	1, 2, FEARG_1,	  ret_string,	f_pathshorten},
     {"perleval",	1, 1, FEARG_1,	  ret_any,
 #ifdef FEAT_PERL
 	    f_perleval