Fix default until-date in other locales

The default until-date used `date` with the system's default locale
as `date` produces localized output by default.
However, `date -d` cannot parse this localized output
but requires a locale independent format:
https://www.gnu.org/software/coreutils/manual/html_node/Options-for-date.html#index-_002dd-19
This sets the locale to a format parsable by `date` as in the article.

git-quick-stats

@@ -27,7 +27,7 @@ _until=${_GIT_UNTIL:-}
 if [[ -n "${_until}" ]]; then
     _until="--until=$_until"
 else
-    _until="--until=$(date '+%a, %d %b %Y %H:%M:%S %Z')"
+    _until="--until=$(LC_TIME=C date '+%a, %d %b %Y %H:%M:%S %Z')"
 fi
 
 # Set files or directories to be excluded in stats