mergetools: don't let meld open all changed files on startup

In meld 3.16, a multi-file change with option '-a' results in
an overview list being opened.
In meld 3.20, a multi-file change with option '-a' results in
an overview list AND every changed file being opened.

Simply removing '-a' seems to work fine for both cases
and also behaves the same as before for single-file changes.

As per the release notes, this is due to the following change
in meld 3.19.0:
"Make the --auto-compare command line flag work again (Kai Willadsen)"

See also https://gitlab.gnome.org/GNOME/meld/-/issues/516

mercurial/defaultrc/mergetools.rc

@@ -107,7 +107,7 b' kdiff3.regkeyalt=Software\\Wow6432Node\\KD'
 
 meld.args=--label=$labellocal $local --label='merged' $base --label=$labelother $other -o $output --auto-merge
 meld.check=changed
-meld.diffargs=-a --label=$plabel1 $parent --label=$clabel $child
+meld.diffargs=--label=$plabel1 $parent --label=$clabel $child
 meld.gui=True
 
 merge.check=conflicts