Add a "Content of selected file" entry to the copy menu for commit files

[stefanhaller]

• PR Description

Some people have the need to get at the file content of a file as it was in an earlier commit. They expect to press e in the commit files view and get the file opened in the state it was in at that commit, somehow. I explained a few times (e.g. in #3902, #4109, and #4327) that this is not how e should work; so if we want this as a feature, we'd need it as a separate command. However, it's a bit tricky to implement; if we check out the selected file to a temp file, it's unclear when to remove the file again. Alternatively we could use EditAndWait to open the file, then we would block until the user closes the editor and delete the file then. This is probably not the best user experience though, e.g. it wouldn't allow opening two files from an older commit at once.

Instead of finding solutions to all these questions, this PR takes a simpler route and adds a command to the "Copy to Clipboard" menu that allows copying the entire content of a file. Users can then open a new, empty editor buffer and paste it in, this is almost as good as opening a temp file, but without all the questions. This was suggested in #4327.

• Please check if the PR fulfills these requirements

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• 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 357c0461	✅ 84.75%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (357c046)	Report Missing	Report Missing	Report Missing
Head commit (f7295a9)	53330	46194	86.62%

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Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#4341)	59	50	84.75%

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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[ChrisMcD1]

I was curious about this method returning the From ref, and discovered RefForAdjustingLineNumberInDiff() just returns the single target commit. Might be a preferable alternative

[stefanhaller]

I did consider this, but that function is for a very specific purpose (and therefore has a very specific name), so it feels wrong to use it here. I think it doesn't make a big difference either way; calling GetFromAndToForDiff and disregarding the from ref also seems clear enough to me.

[ChrisMcD1]

The command log output here is not particularly helpful. It doesn't appear configurable here, but if it could show the file path and refname, that would make more sense? Probably doesn't matter.

[ChrisMcD1]

The command log output here is not particularly helpful. It doesn't appear configurable here, but if it could show the file path and refname, that would make more sense? Probably doesn't matter.

Yeah, that's not configurable. Okay!

lazygit/pkg/commands/oscommands/os.go

Lines 287 to 297 in 19ac926

	func (c *OSCommand) CopyToClipboard(str string) error {
	escaped := strings.Replace(str, "\n", "\\n", -1)
	truncated := utils.TruncateWithEllipsis(escaped, 40)
	msg := utils.ResolvePlaceholderString(
	c.Tr.Log.CopyToClipboard,
	map[string]string{
	"str": truncated,
	},
	)
	c.LogCommand(msg, false)

[jesseduffield]

LGTM, I can see myself using this feature