How do criss-cross merges arise in Git?

Question

I have been going through the, \"git merge-base\", man page and I can\'t understand how multiple merge bases develop. Specifically, I\'m hung up on the following illustratio

Answer 1

Here is a sequence of commits that would generate a criss-cross merge. More steps than the one involving the --amend, but only uses the basics. Run this script to see that you do indeed get a criss-cross merge.

#!/bin/sh
# Initialize 
git init .                                                                                                                       
echo date > file.txt; git add file.txt
git commit -m "Initial commit"

# Make branches                                                                                                                     
git branch A; git branch B

# Make change in A                                                                                                                  
git checkout A
echo date > fileA.txt; git add fileA.txt
git commit -m "first commit along branch A"

# Make change in B; add tag for later                                                                                                               
git checkout B
echo date > fileB.txt; git add fileB.txt
git commit -m "first commit along branch B"
git tag "tag-B"

# Merge A into B (still on B)                                                                                                       
git merge --no-edit A; git tag "M"

# Add another commit on B whose parent is M                                                                                         
echo date > fileB2.txt; git add fileB2.txt
git commit -m "second commit along B"

# Switch to A and add a commit; note that
# M is not an ancestor of this commit                                                                          
git checkout A
echo date > fileA2.txt; git add fileA2.txt
git commit -m "second commit along A"

echo "Best common ancestors (before criss-cross):"
# Should be only one
git merge-base --all A B

# Merge the commit tagged "tag-B" 
# into A generating the cris-cross-merge                                                            
git merge --no-edit tag-B

echo "Best common ancestors (after criss-cross):"
# Should be two
git merge-base --all A B