Bash script that checks if a branch is fast-forwardable, and it its not offers to rebase it for you. Once rebased, the script asks before ff merging.

git-ffable.sh

#!/bin/bash

underline=$(tput sgr 0 1)          # Underline
bold=$(tput bold)             # Bold
normal=$(tput rmso)				# Normal

# Dims
red=$(tput setaf 1) # red
green=$(tput setaf 2) # magenta
blue=$(tput setaf 4) # blue
yellow=$(tput setaf 3) # yellow
magenta=$(tput setaf 5) # magenta
cyan=$(tput setaf 6) # cyan
white=$(tput setaf 7) # white

#Bolds
bred=${txtbld}$(tput setaf 1) # red
bgreen=${txtbld}$(tput setaf 2) # magenta
bblue=${txtbld}$(tput setaf 4) # blue
byellow=${txtbld}$(tput setaf 3) # yellow
bmagenta=${txtbld}$(tput setaf 5) # magenta
bcyan=${txtbld}$(tput setaf 6) # cyan
bwhite=${txtbld}$(tput setaf 7) # white


#Resets
reset=$(tput sgr0)             # Reset



function get_merge_revs {
	MERGE=$(git rev-parse --abbrev-ref $REV);
	MERGE_REV=$(git rev-parse $REV);
	SHORT_MERGE_REV=$(echo $MERGE_REV | cut -c 1-7);
}

function get_orig_merge_revs {
	ORIG_MERGE=$(git rev-parse --abbrev-ref $REV);
	ORIG_MERGE_REV=$(git rev-parse $REV);
	ORIG_SHORT_MERGE_REV=$(echo $MERGE_REV | cut -c 1-7);
}

function get_current_revs {
	CURRENT=$(git rev-parse --abbrev-ref $REV);
	CURRENT_REV=$(git rev-parse $REV);
	SHORT_CURRENT_REV=$(echo $CURRENT_REV | cut -c 1-7);
}

function is_ffable {
	echo ".....";
	git merge-base --is-ancestor $CURRENT_REV $MERGE_REV;
	ffable=$?;

	local A_label="${cyan}${bold}(A)${reset}";
	local B_label="${bmagenta}(B)${reset}";

	if [ $VERBOSE == true ]; then
		echo -e "${A_label}: (${cyan}$CURRENT${reset}) \t[${cyan}$CURRENT_REV${reset}]";
		echo -e "${B_label}: (${magenta}$MERGE${reset}) \t[${magenta}$MERGE_REV${reset}]\n";


		if [  $ffable == 0 ]; then
			echo -e "${bgreen}[*]${reset} ${green}Yes! Revision ${B_label}${green} CAN be Fast-Forward merged into ${A_label}${green}.${reset}"
		else
			echo -e "${bred}[!]${reset} ${red}Well, fuck! Revision ${B_label}${red} can NOT be Fast-Forward merged into ${A_label}${green}!${reset}"
		fi 
	fi
}

function ferge {
	echo "Performing a Fast-Forward merge of $MERGE";
	echo ".....";

	git merge --ff-only $MERGE_REV;
}

# Performs a reverse rebase where the current revision becomes the base for the revision that is passed.
function mebase {

	echo "Checking out $MERGE.....";
 	git checkout --quiet $MERGE;

 	TMP_REV=$(git rev-parse HEAD);

 	# Verify that we have checked out the correct revision
 	if [ "$MERGE_REV" == "$TMP_REV" ]; then

 		echo "Performing REBASE of  ($MERGE) [$SHORT_MERGE_REV] onto ($CURRENT) [$SHORT_CURRENT_REV].....";
 		echo ".....";
		git rebase -i $CURRENT_REV;

		

		#Reset the merge vars
		REV="HEAD";

		#update_merge_branch;
		get_merge_revs;
		get_orig_merge_revs;


		echo "Checkout out your original branch: ($CURRENT) [$SHORT_CURRENT_REV].....";
		git checkout --quiet $CURRENT;
	else
		echo "Checkout of $MERGE seems to have failed."
		exit() { return $1;}
    fi

}

function update_merge_branch {

	if [ "$MERGE" -ne "$BRANCH" ]; then
		git update-ref refs/heads/$BRANCH $REV;
	fi

	echo "[*] Revision ($MERGE) [$SHORT_MERGE_REV] has been properly rebased onto ($CURRENT) [$SHORT_CURRENT_REV] and updated.";

}

function ask_rebase {
	printf "[${bblue}?${reset}] Perform ${underline}${red}rebase${reset}? [Yes/no] : "
	read do_rebase;
	echo ".....";
}

function ask_ferge {
	printf "${bblue}[?]${reset} Perform ${red}merge${reset}? [Yes/no] : "
	read do_ferge;
	echo ".....";
}


echo "${underline}Current${reset} position is [${cyan}$SHORT_CURRENT_REV${reset}] ( ${cyan}$CURRENT${reset})" 


REV=$1;
get_merge_revs;

REV="HEAD";
get_current_revs;

#Backup var for safe keeping after rebases.
BRANCH=$MERGE;

VERBOSE=true;
is_ffable;

if [ "$ffable" -gt 0 ]; then
	ask_rebase;

	if [ $do_rebase == "Yes" ]; then
		mebase;

		echo -e "\nNew revisions (after rebase)"
		is_ffable;
	else
		echo "Nothing left to do...";
		exit() { return $1;}
	fi
fi 


if [ "$ffable" -eq 0 ]; then
	ask_ferge;

	if [ $do_ferge == "Yes" ]; then
		ferge;
	else
		echo "Nothing left to do...";
		exit() { return $1;}
	fi
fi