Expose {{.SelectedCommitRange}} to custom commands

[stefanhaller]

• PR Description

Expose {{.SelectedCommitRange}} to custom commands. It has fields .To and .From (the hashes of the last and the first selected commits, respectively), and it is useful for creating git commands that act on a range of commits.

Fixes #4184.

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[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
Report missing for d7683271	✅ 100.00%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (d768327)	Report Missing	Report Missing	Report Missing
Head commit (4baf008)	52075	45027	86.47%

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 (#4204)	44	44	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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[jesseduffield]

This part:

; the properties above only give you the moving end of the range

Confused me for a bit because I didn't know what it meant. I don't think this part is necessary: it's sufficient to just say we don't support range selection.

[stefanhaller]

I thought it would be good to explain what happens if you access the single-element properties while there's a range selection, since I don't find this entirely obvious. And "moving end of the range" is the only way I could come up with to describe this.

But apparently it didn't work, so I'm happy to take this out if it's more confusing than helpful (9e57418).

Feel free to push a fixup if you want to improve the text further, I'll leave the PR open for a day or two more.

[jesseduffield]

I thought it would be good to explain what happens if you access the single-element properties while there's a range selection, since I don't find this entirely obvious. And "moving end of the range" is the only way I could come up with to describe this.

I think that's an edge case we don't need to worry about.

Code LGTM so feel free to merge

[jesseduffield]

LGTM pending one thing, nice work