Improve the "Find base commit for fixup" command

[stefanhaller]

• PR Description

Improve the ctrl-f command so that it also works when there are no hunks with deleted lines in the diff. This is very useful, for example, when a fixup commit adds a comment to a function that was added in the PR.

Since the exact behavior of the command is getting very complex and hard to understand, I added a design document that describes what it does, and also how it differs from git-absorb which does a very similar thing.

• Please check if the PR fulfills these requirements

• [x] Cheatsheets are up-to-date (run go generate ./...)

• [x] Code has been formatted (see here)

• [x] Tests have been added/updated (see here for the integration test guide)

• [x] Text is internationalised (see here)

• [x] Docs have been updated if necessary

• [x] You've read through your own file changes for silly mistakes etc

[codacy-production[bot]]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
Report missing for f085d10c465c3a80be492390292b787f037c6f91[^1]	:white_check_mark: 89.30%

Coverage variation details

| | Coverable lines | Covered lines | Coverage | | ------------- | ------------- | ------------- | ------------- | | Common ancestor commit (f085d10c465c3a80be492390292b787f037c6f91) | Report Missing | Report Missing | Report Missing | | | Head commit (c9c556beba29f95f7a21703104e92be1aa3e6f08) | 51494 | 43440 | 84.36% | **Coverage variation** is the difference between the coverage for the head and common ancestor commits of the pull request branch: ` - `

Diff coverage details

| | Coverable lines | Covered lines | Diff coverage | | ------------- | ------------- | ------------- | ------------- | | Pull request (#3602) | 187 | 167 | **89.30%** | **Diff coverage** is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: `/ * 100%`

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_{Codacy will stop sending the deprecated coverage status from June 5th, 2024. Learn more} [^1]: Codacy didn't receive coverage data for the commit, or there was an error processing the received data. Check your integration for errors and validate that your coverage setup is correct.