chore: prepare for 6.0.0-a.4 release

Author: wincent

doc/command-t.txt

@@ -573,7 +573,7 @@ POSSIBILITY OF SUCH DAMAGE.
 
 HISTORY                                         *command-t-history*
 
-main (not yet released) ~
+6.0.0-a.4 (5 September 2022) ~
 
 - fix: use correct `git ls-files` switch when `scanners.git.untracked` is `true`.
   (https://github.com/wincent/command-t/pull/405).

lua/wincent/commandt/version.lua

@@ -5,6 +5,6 @@ return {
   major = 6,
   minor = 0,
   patch = 0,
-  prerelease = 'a.3',
-  version = '6.0.0-a.3',
+  prerelease = 'a.4',
+  version = '6.0.0-a.4',
 }