fix: stash before fast-forwarding in a worktree

[EclipseTech]

• Issue: Fast-forward a branch checked out in a worktree ran git merge --ff-only without stashing, so it failed whenever uncommitted changes touched a file that an upstream commit changed
• Fix: Stash around the fast-forward too

[EclipseTech]

• All tests pass
• New test fails w/o fix
• I was not able to test in Windows

[EclipseTech]

How to reproduce bug

Run in an empty directory:

# remote, plus a clone with a worktree for branch "feat"
git init --bare origin.git
git clone origin.git test
cd test
echo "init" > f
git add f && git commit -m 'init'
git push origin
git branch --track feat origin/main
git worktree add ../wt feat
cd ..

# Someone else pushes a change to line 1 of f on feat
git clone origin.git other
cd other
git branch --track feat origin/main
echo 'changes' > f
git commit -am 'upstream' && git push origin main
cd ..

# In the worktree, make an uncommitted edit to line 5 of f
cd wt
echo 'anything' > f

# Run git up
git up

Issue result:

feat    fast-forwarding...
'git merge --ff-only origin/feat' returned exit status 1
error: Your local changes to the following files would be overwritten by merge:
	  f

[msiemens]

Thanks for finding and fixing this!

[msiemens]

This is now released in v2.6.1. Thanks again!