Improve existing git helper scripts

buildroot/share/git/mfinfo

   * ) echo "Usage: `basename $0` [branch]" 1>&2 ; exit 1 ;;
 esac
 
+if [[ $BRANCH == "(no" ]]; then
+  echo "Git is busy with merge, rebase, etc." 1>&2 ; exit 1
+fi
+
 echo "$ORG $FORK $REPO $TARG $BRANCH"

buildroot/share/git/mfinit

 #
 # mfinit
 #
-# Create the upstream repository for Marlin
+# Create the upstream remote for a forked repository
 #
 
-git remote add upstream git@github.com:MarlinFirmware/Marlin.git
+REPO=$(git remote get-url origin 2>/dev/null | sed -E 's/.*\/(.*)\.git/\1/')
+
+if [[ -z $REPO ]]; then
+  echo "`basename $0`: No 'origin' remote found." 1>&2 ; exit 1
+fi
+
+git remote add upstream "git@github.com:MarlinFirmware/$REPO.git"
+
+git fetch upstream

buildroot/share/git/mfnew

 #
 # mfnew
 #
-# Create a new branch based on RCBugFix or dev a given branch name
+# Create a new branch from the default target with the given name
 #
 
 MFINFO=$(mfinfo) || exit
 IFS=' ' read -a INFO <<< "$MFINFO"
 TARG=${INFO[3]}
 
-if [[ ${INFO[4]} == "(no" ]]; then
-  echo "Branch is unavailable!"
-  exit 1
-fi
-
 case "$#" in
-  0 ) BRANCH=pr_for_$TARG-$(date +"%G-%d-%m|%H:%M:%S") ;;
+  0 ) BRANCH=pr_for_$TARG-$(date +"%G-%m-%d_%H.%M.%S") ;;
   1 ) BRANCH=$1 ;;
   * ) echo "Usage: `basename $0` [branch]" 1>&2 ; exit 1 ;;
 esac

buildroot/share/git/mfpr

 TARG=${INFO[3]}
 BRANCH=${INFO[4]}
 
-if [[ $BRANCH == "(no" ]]; then
-  echo "Git is busy with merge, rebase, etc."
-  exit 1
-fi
-
 if [[ ! -z "$1" ]]; then { BRANCH=$1 ; git checkout $1 || exit 1; } fi
 
 if [[ $BRANCH == $TARG ]]; then

buildroot/share/git/mfrb

 MFINFO=$(mfinfo) || exit
 IFS=' ' read -a INFO <<< "$MFINFO"
 
-if [[ ${INFO[4]} == "(no" ]]; then
-  echo "Branch is unavailable!"
-  exit 1
-fi
-
 case "$#" in
   0 ) ;;
   * ) echo "Usage: `basename $0`" 1>&2 ; exit 1 ;;

buildroot/share/git/mfup

 TARG=${INFO[3]}
 OLDBRANCH=${INFO[4]}
 
-if [[ $OLDBRANCH == "(no" ]]; then
-  echo "Branch is unavailable!"
-  exit 1
-fi
-
 case "$#" in
   0 ) BRANCH=$OLDBRANCH ;;
   1 ) BRANCH=$1 ;;