create --progress: show big sizes with more decimals, fixes #3559

[ThomasWaldmann]

Fixes #3559.

At TB scale, the sizes in the borg create --progress status line only changed every 10GB, so
one could not tell whether it was still making progress (the more so when running from cron and
not seeing the beginning of the run).

Formatting

format_file_size (and the sizeof_fmt* helpers) got a fine=False option that gives big sizes
a scale dependent precision, so that changes of ~1MB stay visible:

size	default	fine=True
1234567	1.23 MB	1.23 MB
10**9	1.00 GB	1.000 GB
5000001000000	5.00 TB	5.000001 TB
5*10**15 + 10**6	5.00 PB	5.000000001 PB

Decimals are capped at 9 - a float can not resolve more anyway, and it bounds the output width.

Unit selection is deliberately unchanged (the rounding guard still uses the base precision), so
a size is never shown with a different unit than before. As a consequence, 999999000000 shows
as 0.999999 TB where the default format rounds to 1.00 TB - the value is exact, the unit is
the same. There is a test for it.

Status line

@ThomasWaldmann noted in the issue that more digits shrink the space left for the path, which is
a problem at 80 columns, and that increasing the digits only for much wider terminals would be
the way to go. So:

• on a terminal, the precise format is only used at >= 110 columns. fine adds at most 7 chars
  per size field, i.e. 14 for both, so even in the worst case the path still gets more room than
  it had at 80 columns with the compact format. It is re-evaluated on every refresh, so resizing
  the terminal degrades gracefully.
• if the output does not go to a terminal (e.g. into a logfile), the precise format is always
  used - there is no width limit there.
• the JSON progress output is unchanged (it has raw byte values anyway).

At 80 columns nothing changes:

5.00 TB O 1.57 TB U 362508 N mnt/to_backup/oct/some...her_long_filename_here.bin

At 110 columns, 1MB steps become visible:

5.000000 TB O 1.570000 TB U 362508 N mnt/to_backup/oct/some_rather_long_filename_here.bin
5.000001 TB O 1.570000 TB U 362508 N mnt/to_backup/oct/some_rather_long_filename_here.bin
5.000002 TB O 1.570000 TB U 362508 N mnt/to_backup/oct/some_rather_long_filename_here.bin

--stats / borg info / all other size output keep the default 2 decimals (fine defaults to
False, and Statistics.osize_fmt/usize_fmt are untouched).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.78%. Comparing base (bf6a449) to head (98b8b4f).
⚠️ Report is 5 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #10088   +/-   ##
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  Coverage   86.77%   86.78%           
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  Files          98       98           
  Lines       17277    17285    +8     
  Branches     2622     2623    +1     
=======================================
+ Hits        14992    15000    +8     
  Misses       1587     1587           
  Partials      698      698           

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