patch 9.1.1473: inconsistent range arg for :diffget/diffput

Problem:  inconsistent range arg for :diffget/diffput
Solution: fix the range specification, place the cursor for :diffput and
          :diffget consistently on the last line (Yee Cheng Chin)

Previously, `:<range>diffget` only allowed using 1 or above in the range
value, making it impossible to use the command for a diff block at the
beginning of the file. Fix the range specification so the user can now
use 0 to specify the space before the first line. This allows
`:0,$+1diffget` to work to retrieve all the changes from the other file
instead of missing the first diff block. Also do this for `:diffput`.

Also, make `:diffput` work more similar to `:diffget`. Make it so that
if the cursor is on the last line and a new line is inserted in the
other file, doing `:diffput` will select that diff block below the line,
just like `:diffget` would.

Also clean up the logic a little bit for edge cases and for handling
line matched diff blocks better.

closes: #17579

Signed-off-by: Yee Cheng Chin <ychin.git@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/ex_docmd.c

@@ -4850,7 +4850,8 @@ invalid_range(exarg_T *eap)
 	    case ADDR_LINES:
 		if (eap->line2 > curbuf->b_ml.ml_line_count
 #ifdef FEAT_DIFF
-			    + (eap->cmdidx == CMD_diffget)
+			    + (eap->cmdidx == CMD_diffget ||
+				eap->cmdidx == CMD_diffput)
 #endif
 		   )
 		    return _(e_invalid_range);