Strip the '+' and '-' characters when copying parts of a diff to the clipboard

[stefanhaller]

• PR Description

This makes it easier to copy diff hunks and paste them into code. We only strip the prefixes if the copied lines are either all '+' or all '-' (possibly including context lines), otherwise we keep them. We also keep them when parts of a hunk header is included in the selection; this is useful for copying a diff hunk and pasting it into a github comment, for example.

A not-quite-correct edge case is when you select the '--- a/file.txt' line of a diff header on its own; in this case we copy it as '-- a/file.txt' (same for the '+++' line). This is probably uncommon enough that it's not worth fixing (it's not trivial to fix because we don't know that we're in a header).

Fixes #3859.
Fixes #4511.

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[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
Report missing for ac9b8301	✅ 96.15%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (ac9b830)	Report Missing	Report Missing	Report Missing
Head commit (159bbb0)	55944	48537	86.76%

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 (#4519)	26	25	96.15%

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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Footnotes

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. ↩

[jesseduffield]

I do feel a little uneasy with the implicit conditional logic, but if nobody gets tripped up by this, it does spare us from having to add another keybinding so let's do go with this and see if it confuses anyone