Use CommitterDate instead of AuthorDate, because the latter is not
guaranteed to increase monotonously.

--- vc-orig/git-unix/vc-git.awk
+++ vc/git-unix/vc-git.awk
@@ -39,7 +39,7 @@
 ### Standard encoding is UTF-8.
 				if (Encoding == "") Encoding = "UTF-8"
 ### Extract relevant information from variables.
-				LongDate = substr(AuthorDate, 1, 25)
+				LongDate = substr(CommitterDate, 1, 25)
         DateRAW = substr(LongDate, 1, 10)
         DateISO = DateRAW
         DateTEX = DateISO