json: support BORG_JSON_COMPACT env var for single-line output

[charmikadi]

Description

When BORG_JSON_COMPACT is set, json_dump() produces single-line JSON instead of the default indented output. This makes it easier for log collectors to parse borg's JSON stdout as one event per line. Implementation uses an env var and not CLI flag.

Fixes #3605

Checklist

• PR is against master
• New code has tests and docs where appropriate
• Tests pass (run tox or the relevant test subset)
• Commit messages are clean and reference related issues

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.13%. Comparing base (5aaa2dd) to head (c322dd5).
⚠️ Report is 9 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9855      +/-   ##
==========================================
+ Coverage   85.11%   85.13%   +0.01%     
==========================================
  Files          93       93              
  Lines       15408    15421      +13     
  Branches     2326     2329       +3     
==========================================
+ Hits        13114    13128      +14     
+ Misses       1596     1595       -1     
  Partials      698      698              

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[Goddesen]

Maybe not directly relevant to this PR, but might as well put it here: JSON output is meant to be read programmatically, right? Why isn't it default to output the most compact form possible to spare the extra work? Users can set the indent envvar or pipe into jq for readable output if that's what they want.

[ThomasWaldmann]

@Goddesen good idea, guess we could change the default for borg2.

[ThomasWaldmann]

@charmikadi Thanks!

Can you please do a backport to 1.4-maint branch?