Checkout this approach for knowing what to transfer. This revision uses a local tag to track each time you use the command. The first time you use it, it pushes all files in the project. At the end of each run it updates the tag to point to HEAD. Next time you run it, it will only push files that have changed since the last push.

git-transmit

#!/usr/bin/env bash
# author: Thomas Aylott SubtleGradient.com
# author: Nathan Nobbe quickshiftin.com

# Find out where HEAD is pointing
head_ref=$(git show-ref --head -s | head -n 1)
# Check to see if transmit tag exists, and get transmit tag hash
_transmit_ref=$(git show-ref --verify -s refs/tags/transmit)

# If there's not transmit tag, create it for the first time.
if [ $? -gt 0 ]; then
    # This won't work well for projects with multiple roots,
    # but for the typical case it will find the first commit
    # and tag that for the initial push.
    first_commit=$(git rev-list --max-parents=0 HEAD)
    echo 'Creating initial transmit tag'
    git tag -m 'Creating transmit tag' transmit $first_commit
fi

# Find out where the tag is pointing
# @note Probably a cleaner way to do this ...
transmit_ref=$(git cat-file tag $_transmit_ref | head -n 1 | sed 's/^object //')

# If the transmit tag is the same commit as HEAD,
# there's nothing to do
if [ "$transmit_ref" == "$head_ref" ]; then
    echo 'No changes to push'
    exit 0
fi

# Iterate over all the files that changed, adding then to Transmit 
echo 'Transmitting'
for i in $(git diff --name-only transmit HEAD); do
    if [[ -e "$i" ]]; then
        echo "Adding $i to Transmit"
        open -a Transmit "$i"
    fi
done

# Update the tag so we know where the last push occurred
echo 'Updating transmit tag'
git tag -d transmit
git tag -m 'Updating transmit tag' transmit HEAD

Same problem as @reibejoy, solved using /usr/local/sbin/ instead.