Use git reset instead of git pull to avoid attempting a merge commit

[andersk]

Summary

Use git reset instead of git pull to avoid attempting a merge commit. Also, use git remote set-head in case the repository’s default branch has been renamed.

Fixes #5303.

Test plan

Tested with this reproduction script:

#!/usr/bin/env bash
set -eux
tmpdir="$(mktemp -dt)"
trap 'rm -rf "$tmpdir"' EXIT
cd "$tmpdir"

git init some-library
cd some-library
echo '{"name": "some-library", "version": "1.0.0"}' > package.json
git add package.json
git commit -am 'Initial commit'
echo '{"name": "some-library", "version": "1.0.1"}' > package.json
git commit -am 'Bump version'
some_library="git+file://$PWD"

mkdir ../my-app
cd ../my-app
echo '{}' > package.json
XDG_CONFIG_HOME="$tmpdir" yarn add "$some_library"

cd ../some-library
echo '{"name": "some-library", "version": "1.0.2"}' > package.json
git commit --amend -am 'Force push'

# Uncomment to test branch rename:
#git branch -m develop

cd ../my-app
XDG_CONFIG_HOME="$tmpdir" yarn add "$some_library"

Without this fix, the second yarn add fails with:

+ yarn add git+file:///tmp/tmp.8JjoQJZXLa/some-library
yarn add v1.22.10
warning package.json: No license field
warning No license field
[1/4] Resolving packages...
error Command failed.
Exit code: 128
Command: git
Arguments: pull
Directory: /home/anders/.cache/yarn/v6/.tmp/ea609cd4e4a2249a3f41d450a33f8312
Output:
hint: Pulling without specifying how to reconcile divergent branches is
hint: discouraged. You can squelch this message by running one of the following
hint: commands sometime before your next pull:
hint: 
hint:   git config pull.rebase false  # merge (the default strategy)
hint:   git config pull.rebase true   # rebase
hint:   git config pull.ff only       # fast-forward only
hint: 
hint: You can replace "git config" with "git config --global" to set a default
hint: preference for all repositories. You can also pass --rebase, --no-rebase,
hint: or --ff-only on the command line to override the configured default per
hint: invocation.
Committer identity unknown

*** Please tell me who you are.

Run

  git config --global user.email "you@example.com"
  git config --global user.name "Your Name"

to set your account's default identity.
Omit --global to set the identity only in this repository.

fatal: unable to auto-detect email address (got 'anders@virtual-fork.(none)')
info Visit https://yarnpkg.com/en/docs/cli/add for documentation about this command.

Even if one provides the requested Git configuration first, it still fails:

+ yarn add git+file:///tmp/tmp.bxRQFzTmbn/some-library
yarn add v1.22.10
warning package.json: No license field
warning No license field
[1/4] Resolving packages...
error Command failed.
Exit code: 1
Command: git
Arguments: pull
Directory: /home/anders/.cache/yarn/v6/.tmp/ed8d0fef0fcae909d28ca542249075a8
Output:
hint: Pulling without specifying how to reconcile divergent branches is
hint: discouraged. You can squelch this message by running one of the following
hint: commands sometime before your next pull:
hint: 
hint:   git config pull.rebase false  # merge (the default strategy)
hint:   git config pull.rebase true   # rebase
hint:   git config pull.ff only       # fast-forward only
hint: 
hint: You can replace "git config" with "git config --global" to set a default
hint: preference for all repositories. You can also pass --rebase, --no-rebase,
hint: or --ff-only on the command line to override the configured default per
hint: invocation.
Auto-merging package.json
CONFLICT (content): Merge conflict in package.json
Automatic merge failed; fix conflicts and then commit the result.
info Visit https://yarnpkg.com/en/docs/cli/add for documentation about this command.

With this fix, it works correctly with or without the Git configuration:

+ yarn add git+file:///tmp/tmp.IehTe8fiZv/some-library
yarn add v1.23.0-0
warning package.json: No license field
warning No license field
[1/4] Resolving packages...
[2/4] Fetching packages...
[3/4] Linking dependencies...
[4/4] Building fresh packages...
success Saved lockfile.
warning No license field
success Saved 1 new dependency.
info Direct dependencies
└─ some-library@1.0.2
info All dependencies
└─ some-library@1.0.2
Done in 0.27s.