patch 8.1.0632: using sign group names is inefficient

Problem:    Using sign group names is inefficient.
Solution:   Store group names in a hash table and use a reference to them.
            Also remove unnecessary use of ":exe" from the tests.  (Yegappan
            Lakshmanan, closes #3715)

src/structs.h

@@ -733,6 +733,17 @@ typedef struct proptype_S
 
 
 #if defined(FEAT_SIGNS) || defined(PROTO)
+// Sign group
+typedef struct signgroup_S
+{
+    short_u	refcount;		// number of signs in this group
+    char_u	sg_name[1];		// sign group name
+} signgroup_T;
+
+// Macros to get the sign group structure from the group name
+#define SGN_KEY_OFF	offsetof(signgroup_T, sg_name)
+#define HI2SG(hi)	((signgroup_T *)((hi)->hi_key - SGN_KEY_OFF))
+
 typedef struct signlist signlist_T;
 
 struct signlist
@@ -740,7 +751,7 @@ struct signlist
     int		id;		/* unique identifier for each placed sign */
     linenr_T	lnum;		/* line number which has this sign */
     int		typenr;		/* typenr of sign */
-    char_u	*group;		/* sign group */
+    signgroup_T	*group;		/* sign group */
     int		priority;	/* priority for highlighting */
     signlist_T	*next;		/* next signlist entry */
     signlist_T  *prev;		/* previous entry -- for easy reordering */