cli/command/formatter: assorted fixes and cleanups

[thaJeztah]

cli/command/formatter: optimize ContainerContext.Names

Optimize formatting of container name(s);

• Inline StripNamePrefix in the loop, so that we don't have to
  construct a new slice with names (in most cases only to pick
  the first one).
• Don't use strings.Split, as it allocates a new slice and we only
  used it to check if the container-name was a legacy-link (contained
  slashes).
• Use a string-builder to concatenate names when not truncating instead
  of using an intermediate slice (and strings.Join).

cli/command/formatter: Context.postFormat: remove redundant buffer

A tabwriter is backed by a buffer already, because it needs to re-flow columns
based on content written to it. This buffer was added in moby@ea61dac9e6 as
part of a new feature to allow for custom delimiters; neither the patch, nor
code-review on the PR mention the extra buffer, so it likely was just overlooked.

This patch;

• removes the redundant buffer
• adds an early return for cases where no tabwriter is used.

cli/command/formatter: add Format.templateString, remove Context.preFormat

The Context.preFormat method normalizes the Format as given by the user,
and handles (e.g.) stripping the "table" prefix and replacing the "json"
format for the actual format ({{json .}}).

The method used a finalFormat field on the Context as intermediate,
and was required to be called before executing the format.

This patch adds a Format.templateString() method that returns the
parsed format instead of storing it on the Context. It is currently
not exported, but something we could consider in future.

cli/command/formatter: NewStats: update GoDoc and add TODO

Update the GoDoc to better align with the actual implementation. The
"idOrName" is used for fuzzy-matching the container, which can result
in multiple stats for the same container:

docker ps --format 'table {{.ID}}\t{{.Names}}'
CONTAINER ID   NAMES
b49e6c21d12e   quizzical_maxwell

docker stats --no-stream quizzical_maxwell b49e6c21d12e b49e6
CONTAINER ID   NAME                CPU %     MEM USAGE / LIMIT     MEM %     NET I/O           BLOCK I/O        PIDS
b49e6c21d12e   quizzical_maxwell   0.10%     140.8MiB / 7.653GiB   1.80%     3.11MB / 13.4kB   115MB / 1.12MB   28
b49e6c21d12e   quizzical_maxwell   0.10%     140.8MiB / 7.653GiB   1.80%     3.11MB / 13.4kB   115MB / 1.12MB   28
b49e6c21d12e   quizzical_maxwell   0.10%     140.8MiB / 7.653GiB   1.80%     3.11MB / 13.4kB   115MB / 1.12MB   28

We should resolve the canonical ID once, then use that as reference
to prevent duplicates. Various parts in the code compare Container
against "ID" only (not considering "name" or "ID-prefix").

- Human readable description for the release notes

- A picture of a cute animal (not mandatory but encouraged)

[codecov-commenter]

Codecov Report

❌ Patch coverage is 81.08108% with 7 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cli/command/formatter/container.go	72.72%	2 Missing and 1 partial ⚠️
cli/command/container/formatter_stats.go	0.00%	2 Missing ⚠️
cli/command/formatter/formatter.go	91.66%	1 Missing and 1 partial ⚠️

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