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>
diff --git a/runtime/doc/diff.txt b/runtime/doc/diff.txt
index 8156466..493c740 100644
--- a/runtime/doc/diff.txt
+++ b/runtime/doc/diff.txt
@@ -1,4 +1,4 @@
-*diff.txt*      For Vim version 9.1.  Last change: 2025 Mar 28
+*diff.txt*      For Vim version 9.1.  Last change: 2025 Jun 20
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -304,18 +304,20 @@
 
 
 When no [range] is given, the diff at the cursor position or just above it is
-affected.  When [range] is used, Vim tries to only put or get the specified
-lines.  When there are deleted lines, this may not always be possible.
+affected.  There can be deleted lines below the last line of the buffer.  When
+the cursor is on the last line in the buffer and there is no diff above this
+line, and no [range] is given, the diff below the cursor position will be used
+instead.
 
-There can be deleted lines below the last line of the buffer.  When the cursor
-is on the last line in the buffer and there is no diff above this line, the
-":diffget" and "do" commands will obtain lines from the other buffer.
+When [range] is used, Vim tries to only put or get the specified lines.  When
+there are deleted lines, they will be used if they are between the lines
+specified by [range].
 
-To be able to get those lines from another buffer in a [range] it's allowed to
-use the last line number plus one.  This command gets all diffs from the other
-buffer: >
+To be able to put or get those lines to/from another buffer in a [range] it's
+allowed to use 0 and the last line number plus one.  This command gets all
+diffs from the other buffer: >
 
-	:1,$+1diffget
+	:0,$+1diffget
 
 Note that deleted lines are displayed, but not counted as text lines.  You
 can't move the cursor into them.  To fill the deleted lines with the lines