vim-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 vim/vim#3715)
7a2d9892b7

src/nvim/sign_defs.h

@@ -5,6 +5,17 @@
 
 // signs: line annotations
 
+// Sign group
+typedef struct signgroup_S
+{
+    uint16_t	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
@@ -12,7 +23,7 @@ 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