A collection of useful Git tricks
If two Git branches:
- share some cherry-picked commits but also differ
- are based on different base branches
...it can be useful to generate a diff as if they were to land on the same target (eg. main). Using synthetic trees with git merge-tree (Git >= 2.38), shared patches cancel out, so the output highlights only what's unique to each range.
Add the following function to your .zshrc, .bash_profile, .bashrc, etc:
# compare_changesets: compare two change-sets on a common target (Git >= 2.38)
# ----------------------------------------------------------------------------
# API:
# compare_changesets <TARGET> <BASE_A> <TIP_A> <BASE_B> <TIP_B>
#
# Example:
# compare_changesets main production production-login-ui main login-ui
#
# Diagram & explanation:
#
# C1' C2' D1 D2
# production-login-ui o------o------o-----o
# /
# production o--o--o
# /
# main - - ---o
# \
# login-ui o------o
# C1 C2
#
# - Shared patches: C1/C2 on login-ui and C1'/C2' on production-login-ui represent
# the same logical changes (eg. cherry-picked or equivalent patches). When we
# synthesize both change-sets onto TARGET (main), those shared patches produce the
# same resulting content on both synthetic trees and therefore cancel out of the
# comparison.
#
# - What the diff shows: with A = (production → production-login-ui) and
# B = (main → login-ui) compared on TARGET = main, the resulting `git diff -M`
# between the two synthetic trees highlights only what is unique between the
# change-sets on that common base. In this scenario, that is D1 and D2 — eg.
# the additional changes after C2' on production-login-ui (modulo any conflict
# resolutions that could introduce minor context differences).
compare_changesets() {
set -o nounset
if [ "$#" -ne 5 ]; then
echo "Usage: compare_changesets <TARGET> <BASE_A> <TIP_A> <BASE_B> <TIP_B>" >&2
return 2
fi
local target="$1" base_a="$2" tip_a="$3" base_b="$4" tip_b="$5"
# Build synthetic trees by merging each change-set onto target with its own base
local tree_a
tree_a="$(git merge-tree --write-tree --merge-base "$base_a" "$target" "$tip_a")"
local tree_b
tree_b="$(git merge-tree --write-tree --merge-base "$base_b" "$target" "$tip_b")"
# Show the net difference between the two effects on target
git diff -M "$tree_a" "$tree_b"
}