MFC r265842,r266120,r266121,r266959,r267148,r269985,r281789,r282261,r285064:

r265842 (by eadler):

arcconfig: add one

Add a .arcconfig to allow arc to work in its usual way.

r266120 (by bapt):

Add project name to the arc config it is still required when arcanist is used with SVN

r266121 (by bapt):

Add the missing coma

r266959 (by eadler):

arc: add linting for python files

r267148 (by bapt):

Prevent arc commands from overwriting history

r269985 (by gjb):

Update the URL to the phabricator instance.

Sponsored by: The FreeBSD Foundation

r281789 (by eadler):

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

r282261 (by eadler):

Phabricator: enable "history.immutable":
With certain arc workflows leaving history.immutable as false
results in using the upstream template instead of our usual
commit template. Since the git workflow issues alluded to in my
prior commit message can be worked around, set history.immutable
once again.

r285064 (by mat):

Add repository.callsign, to help arcanist figure out what repo it's
doing things on when not using Subversion.

With hat: phabric-admin@
Sponsored by: Absolight