Don't preserve commit message when it's unchanged from initial message

[stefanhaller]

• PR Description

Sometimes we populate the commit message panel with a pre-created commit
message. The two cases where this happens is:

• you type w to commit, in which case we put the skipHookPrefix in the subject
• you have a commitPrefix pattern, in which case we match it against the branch
  name and populate the subject with the replacement string if it matches

In either case, if you have a preserved commit message, we use that.

Now, when you use either of these and then cancel, we preserve that initial,
unchanged message and reuse it the next time you commit. This has two problems:
it strips spaces, which is a problem for the commitPrefix patterns, which often
end with a space. And also, when you change your config to experiment with
commitPrefix patterns, the change seemingly doesn't take effect, which can be
very confusing.

To fix both of these problems, only preserve the commit message when it is not
identical to the initial message.

• Please check if the PR fulfills these requirements

• Cheatsheets are up-to-date (run go generate ./...)
• Code has been formatted (see here)
• Tests have been added/updated (see here for the integration test guide)
• Text is internationalised (see here)
• If a new UserConfig entry was added, make sure it can be hot-reloaded (see here)
• Docs have been updated if necessary
• You've read through your own file changes for silly mistakes etc

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
Report missing for e29ddf41	✅ 100.00%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (e29ddf4)	Report Missing	Report Missing	Report Missing
Head commit (3a30211)	51441	44418	86.35%

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#4110)	23	23	100.00%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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