GIT - how to keep a file common across all branches

Question

Suppose I have a file \"ChangeLog\" in branch \"master\". I want to record information about all changes in any branch in this file (in more detail than in a commit message

Answer 1

You could use a post-checkout hook:

#!/bin/sh

newref="$2"
isbranch="$3"

# ignore file checkouts
if test $isbranch -eq 0; then
  exit 0
fi

path=ChangeLog

git for-each-ref --sort=-committerdate \
                 --format='%(objectname) %(refname:short)' \
                 refs/heads/\* |
while read sha ref; do
  if git rev-parse --verify --quiet "$sha:$path" >/dev/null
  then
    if test "$newref" != "$sha"; then
      echo Checking out $path from $ref
      git checkout $sha -- "$path"
    fi
    break
  fi
done

It will still be up to you to add and commit the ChangeLog on your branch if appropriate.

This hook sorts branches by their heads' respective commit dates, which could be problematic. Say you create a topic branch rooted at a commit in master's history. Even though its snapshot of ChangeLog is older in the sense of calendar time, the hook above will treat it as the newest because it is referenced by a commit that was created more recently, so be careful that you don't accidentally lose work by switching branches when you have unstaged changes to ChangeLog.