add: cleanup and improve output

[shcheklein]

• tidy up remaining stats (no need for <00:00, add total size in bytes, etc)
• add progress to other uses of stage.dump
• profile repo.check_modified_graph(), stage.save(), stage.dump() etc to find out what other functions are slow, perhaps add progress for them too
  • md5sums only take about 10% of the total time. There must be other more time consuming functions
• make sure we don't show an extra progress bar when there is only one target (majority of cases, see below)
• cleanup output when if fails add: cleanup and improve output #2699 (comment)
• large dirs progress bar hanging when collecting files before computing checksums progress when collecting directories with giant number of files #2770 add: cleanup and improve output #2699 (comment)
• make sure that we show HINT message to enable cache optimization properly when copy is taking too long already

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Extra progress bar concern:

We started using an extra progress bar that is counting targets, as described and shown here: #2658 . In most cases though there is a single target - dvc add file or dvc add directory. That extra progress bar is not needed in those cases. It's needed in case of multiple targets or if -R option is specified.

[shcheklein]

Also, when if fails to add the output is also confusing:

100%|██████████|Add                                                  1/1 [00:02<00:00,  2.86s/file]
ERROR: output 'n01667114' does not exist

Failure or success or both?

[shcheklein]

Also on a large directory it looks like it hangs for a long time on:

  0%|          |Add                             0/1 [00:00<?,     ?file/s]

not clear what is it doing inside before running the Computing hashes phase.

it is also stuck on that single line after the computing hashes phase.

[Suor]

+1 on final message and the rest.

We never had systematic approach to UI, this is the core reason of all of these.

[shcheklein]

💯 on We never had systematic approach to UI, this is the core reason of all of these. 🙏

[shcheklein]

Also, it looks like on Windows dvc add does not print anything at all for a long period of time. I don't see any progress bars whatsoever.

[shcheklein]

Created a separate ticket to deal with the missing output on Windows - #2747

[efiop]

Also, would probably have to add a progress bar for RemoteBase._save_dir since it takes quite a while on dirs with giant number of files.