patch 8.1.2231: not easy to move to the middle of a text line

Problem:    Not easy to move to the middle of a text line.
Solution:   Add the gM command. (Yasuhiro Matsumoto, closes #2070)

runtime/doc/index.txt

@@ -781,6 +781,7 @@ tag		char	      note action in Normal mode	~
 |gn|		gn	      1,2  find the next match with the last used
 				   search pattern and Visually select it
 |gm|		gm		1  go to character at middle of the screenline
+|gM|		gM		1  go to character at middle of the text line
 |go|		go		1  cursor to byte N in the buffer
 |gp|		["x]gp		2  put the text [from register x] after the
 				   cursor N times, leave the cursor after it

runtime/doc/motion.txt

@@ -1,4 +1,4 @@
-*motion.txt*    For Vim version 8.1.  Last change: 2019 Jun 02
+*motion.txt*    For Vim version 8.1.  Last change: 2019 Oct 28
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -227,6 +227,12 @@ g^			When lines wrap ('wrap' on): To the first non-blank
 gm			Like "g0", but half a screenwidth to the right (or as
 			much as possible).
 
+							*gm* *gM*
+gM			Like "g0", but to halfway the text of the line.
+			With a count: to this percentage of text in the line.
+			Thus "10gM" is near the start of the text and "90gM"
+			is near the end of the text.
+
 							*g$* *g<End>*
 g$ or g<End>		When lines wrap ('wrap' on): To the last character of
 			the screen line and [count - 1] screen lines downward

runtime/doc/quickref.txt

@@ -47,6 +47,7 @@ N is used to indicate an optional count that can be given before the command.
 |g$|	N  g$		to last character in screen line (differs from "$"
 			   when lines wrap)
 |gm|	   gm		to middle of the screen line
+|gM|	   gM		to middle of the line
 |bar|	N  |		to column N (default: 1)
 |f|	N  f{char}	to the Nth occurrence of {char} to the right
 |F|	N  F{char}	to the Nth occurrence of {char} to the left

runtime/doc/usr_25.txt

@@ -346,12 +346,13 @@ scroll:
 
 	g0		to first visible character in this line
 	g^		to first non-blank visible character in this line
-	gm		to middle of this line
+	gm		to middle of screen line
+	gM		to middle of the text in this line
 	g$		to last visible character in this line
 
-		|<--	 window    -->|
-	some long    text, part of which is visible ~
-		 g0  g^    gm	     g$
+		|<--	  window     -->|
+	some long    text, part of which is visible in one line ~
+		 g0  g^    gm	   gM g$
 
 
 BREAKING AT WORDS				*edit-no-break*