tests: Load screendump files with "git vimdumps"

Also, fold the difference part and the bottom part for the
identical screendump files.

closes: #16380

Co-authored-by: D. Ben Knoble <ben.knoble+github@gmail.com>
Signed-off-by: Aliaksei Budavei <0x000c70@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/doc/tags

@@ -7926,6 +7926,7 @@ gf	editing.txt	/*gf*
 gg	motion.txt	/*gg*
 gh	visual.txt	/*gh*
 gi	insert.txt	/*gi*
+git-vimdumps	terminal.txt	/*git-vimdumps*
 gj	motion.txt	/*gj*
 gk	motion.txt	/*gk*
 glob()	builtin.txt	/*glob()*

runtime/doc/terminal.txt

@@ -494,6 +494,12 @@ ConPTY problems have been fixed "winpty" will be preferred.
 Environment variables are used to pass information to the running job:
     VIM_SERVERNAME	v:servername
 
+							*git-vimdumps*
+There exists a git-difftool extension called `git-vimdumps` that can be used
+to conveniently inspect screendump files and diff them.  Please see in the Vim
+Repository the file `src/testdir/commondumps.vim` on how to create and use
+this git extension.
+
 ==============================================================================
 2. Terminal functions				 *terminal-function-details*
 

runtime/syntax/testdir/README.txt

@@ -199,6 +199,9 @@ You can now examine the extracted screendumps:
 Viewing generated screendumps (submitted for a pull request)
 ------------------------------------------------------------
 
+Note: There is also a "git difftool" extension described in
+      src/testdir/commondumps.vim
+
 First, you need to check out the topic branch with the proposed changes and
 write down a difference list between the HEAD commit (index) and its parent
 commit with respect to the changed "dumps" filenames: