doc/README

cleanup: use clang-tidy to add missing {} around one-line statements

I find this easier to read. Cleanup performed like this:

hg files 'set:(**.c or **.cc or **.h) and not "listfile:contrib/clang-format-ignorelist"' | while read f ; do
  clang-tidy -fix -checks=readability-braces-around-statements $f -- $(python-config --cflags) -Imercurial/cext -Imercurial
done
make format-c

I had to revert chg/chg.c as it's got a construct that seems to confuse
clang-tidy, so I'll work on that file later if this change is acceptable. I
only tackle files that are under clang-format's authority because otherwise
I'd have to do a bunch of manual formatting. A few files didn't get edited
because clang-tidy couldn't find some headers. Again, I'll figure that out
later assuming this change is accepted.

No check-code rule added for now because writing the regex sounds hard. In a
perfect world I guess we could write a test that uses clang-tidy on these
files, but I think clang-tidy is pretty rarely installed. :/

Differential Revision: https://phab.mercurial-scm.org/D5675

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