p4merge is a free visual diff tool for Mac OS X that can be used as a git mergetool. This allows you to resolve some more complicated merge conflicts. Here is a quick getting started guide.

.gitconfig

# Use p4merge as a mergetool for visually resolving Git merge conflicts
# by adding these lines to ~/.gitconfig

[merge]
    conflictstyle = diff3
    tool = p4merge
[mergetool "p4merge"]
    cmd = /Applications/p4merge.app/Contents/Resources/launchp4merge "$PWD/$BASE" "$PWD/$REMOTE" "$PWD/$LOCAL" "$PWD/$MERGED"
    trustExitCode = false
[mergetool]
    prompt = false
    keepBackup = false

# Download p4merge (free) from http://cdist2.perforce.com/perforce/r14.1/bin.macosx106x86/P4V.dmg
# To resolve a merge conflict, type:
#   git mergetool [filespec]
# To see changes in a common diff, type:
#   git diff --staged
# To back out a merge attempt:
#   git merge --abort
# To remove any dirt left behind
#   git clean -f --dryrun
# then remove the --dryrun if all looks legit.

# Crash course in p4merge:
# As you scroll down, you'll see three shapes to the right of each hunk in the bottom output pane.
# Shapes correspond to LOCAL (current branch), REMOTE (what you're merging in), and BASE (common ancestor).
# Click a shape to toggle inclusion in the bottom output pane.
# Only light up ONE of the shapes! 
# If you're backmerging master into a feature branch, you'll probably prefer REMOTE before BASE before LOCAL.
# When done, save and exit, then git will recognize what you've saved. You can git diff --staged