Change keybinding of "keep" item in Merge Conflict menu back to 'k'

[stefanhaller]

Now that we can use 'k' as a menu item binding (this was fixed in #5131), use it for the "keep" entry in the merge menu. I don't think this will be a problem for people's muscle memory, given that this menu is not encountered every day; and it's simply the better keybinding.

This reverts commit b32b552.

[stefanhaller]

FYI @zingazzi

[codacy-production]

Coverage summary from Codacy

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Coverage variation	Diff coverage
Report missing for 70fe7bd1	✅ ∅

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (70fe7bd)	Report Missing	Report Missing	Report Missing
Head commit (6043eb5)	59289	51592	87.02%

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 (#5132)	0	0	∅ (not applicable)

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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