git-diffdiff

#!/bin/bash

usage()
{
    self=$( basename $0 )
    cat << EOUSAGE
Usage: $self base1 branch1 [[base2] branch2] [-- <extra "git diff" params>]

Compares the overall change sets for two pairs of commits/branches.
Similar to range-diff, except that range-diff compares per-commit.
This compares everything at once.

Can be run with 2 to 4 branch parameters.

With 4, the differences between the first pair and the second pair are
shown. For both pairs, the "branch" is compared against the merge base
for "base" and "branch" so you don't have to find it.

Specifically, "$self A B C D" is equivalent to
"diff -u -minimal <( git diff --merge-base A B )
    <( git diff --merge-base C D )"

With 3, the first parameter is used as the "base" branch for both of the
other two.

Thus "$self A B D" is equivalent to "$self A B A D".

With 2 branches, each branch is compared against their common base.
Similar to "git range-diff A...B", except I don't want to write a parser
right now.

Thus "$self A B" is equivalent to "$self B A A B". The ordering assumes
that A is the "older" branch, and so the comparison will treat its
changes as the diff's "file1".
EOUSAGE

    exit 1
}

if [[ $# -lt 2 ]]
then
    usage
fi

bcount=0

for f in "$@"
do
    if [[ "$f" == "--" ]]
    then
        break
    fi
    bcount=$[ $bcount + 1 ]
done

if [[ $bcount -lt 2 || $bcount -gt 4 ]]
then
    usage
fi

base1=$1
shift
branch1=$1
shift
if [[ $bcount -eq 2 ]]
then
    # Use both branches as base and branch, swapping the apparent order
    base2=$base1
    branch2=$branch1

    base1=$branch2
    branch1=$base2
else
    if [[ $bcount -eq 3 ]]
    then
        base2=$base1
    else
        base2=$1
        shift
    fi
    branch2=$1
    shift
fi

diffmb()
{
    git diff --merge-base "$@"
}

if [[ -t 1 ]]
then
    diffparam=( "--color=always" )
fi

diff -u --minimal "${diffparam[@]}" \
    <( diffmb $base1 $branch1 "$@" ) \
    <( diffmb $base2 $branch2 "$@" ) | less