arc lint: ignore /tests/ in chmod

shell scripts in scripts don't need
to be chmod +x to work. In fact most are not.
Of the tests I found from a simple search:
65 are chmod +x
84 are chmod -x

simply disable the check for test shell scripts.

Recommit requested by: cem, rgrimes

arclint: revert in prep for recommitting

arc lint: Explain linting explain

shell scripts in scripts don't need
to be chmod +x to work. In fact most are not.
Of the tests I found from a simple search:
65 are chmod +x
84 are chmod -x

simply disable the check for test shell scripts.

sponge(1): initial commit

sponge(1) is a utility that reads input until
complete, then opens the output file, then
writes to it. This makes it useful in pipelines
that read and write to the same file.

Reviewed by: wblock, jilles, imp, cem, danfe (all: various iterations)
Inspired by: https://joeyh.name/code/moreutils/

phabricator related changes:
- don't lint either contrib or crypto: these are both externally written
directories
- add additional linters for spelling (check common typos like teh ->
the)
- chmod linter checks for executible bit on bad files
- merge-conflict checks for merge conflict tokens then may have been
resolved incorrectly
- filename checks for back characters in filenames
- json for json syntax correctness

- remove history.immutable: it is meaningless on subversion, and causes
workflow problems when trying to use git. It it set to 'true' by
default with hg

arc: add linting for python files