fix(chain): simplify Append::append impl for keychain::ChangeSet

[evanlinjin]

We only need to loop though entries of other. The logic before was wasteful because we were also looping though all entries of self even if we do not need to modify the self entry.

Description

Notes to the reviewers

Changelog notice

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

Bugfixes:

• This pull request breaks the existing API
• I've added tests to reproduce the issue which are now passing
• I'm linking the issue being fixed by this PR

[coveralls]

Pull Request Test Coverage Report for Build 8951617544

Details

• 10 of 10 (100.0%) changed or added relevant lines in 1 file are covered.
• 353 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-2.2%) to 83.381%

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Files with Coverage Reduction	New Missed Lines	%
crates/bdk/src/descriptor/mod.rs	5	92.45%
crates/bdk/src/descriptor/dsl.rs	348	56.35%

Totals
Change from base Build 8837790333:	-2.2%
Covered Lines:	10521
Relevant Lines:	12618

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💛 - Coveralls