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progress: avoid ui.configbool() lookup when progress bar is active

Authored by indygreg.

Description

progress: avoid ui.configbool() lookup when progress bar is active

Profiling revealed that the ui.configbool('progress', 'debug') during
progress bar updates was consuming a significant amount of overhead.

This commit adds an attribute on progress bar instances that caches
this config option.

The impact on hg perfprogress with default options is significant:

before: ! wall 4.641942 comb 4.580000 user 4.210000 sys 0.370000 (best of 3)
after: ! wall 1.948626 comb 1.950000 user 1.950000 sys 0.000000 (best of 5)

After this change, profiling reveals that progress.progbar.progress()
is now consuming ~73% of time.

This change does not improve the execution time if the progress bar
is disabled. We may want a more comprehensive solution for that case,
as the progress bar won't be enabled in a number of scenarios (e.g.
servers and processes not attached to an interactive TTY).

I also think that overhead of ~2.0s for 1M updates is a bit high.
I suspect further refactoring of the progress bar can significantly
reduce overhead. I don't have plans to do this, however.

Differential Revision: https://phab.mercurial-scm.org/D5408

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