Fix style_file() so it works for a vector of files from different directories

[lorenzwalthert]

Closes #521 by not changing working directory to directory where the file lays. The bug was long undetected because setwd() fails silently on a character vector within withr::with_dir() and is particularly relevant for #467, e.g. when R files in R/ and tests/testthat are staged simultaneously.

Note that this is an API change in the sense that now, the path to the directory where the files lay are printed, not just the file names, which also affects the invisible return values of the stylers that contain the path.

withr::with_dir(
  "~/datasciencecoursera/styler", {
    out <- styler::style_file(c("R/utils-files.R", "tests/testthat/strict/strict-out.R"))
    out
  }
)
#> Styling  2  files:
#>  R/utils-files.R                    ✔ 
#>  tests/testthat/strict/strict-out.R ✔ 
#> ────────────────────────────────────────
#> Status   Count   Legend 
#> ✔    2   File unchanged.
#> ℹ    0   File changed.
#> ✖    0   Styling threw an error.
#> ────────────────────────────────────────
#> # A tibble: 2 x 2
#>   file                               changed
#>   <chr>                              <lgl>  
#> 1 R/utils-files.R                    FALSE  
#> 2 tests/testthat/strict/strict-out.R FALSE

^{Created on 2019-06-19 by the reprex package (v0.2.1)}

This change has implications for testing also. To make a comparison with an exact expectation, in the tests, we temporarily change the working directory to keep file paths and ruler width (that depends on the path) identical on all platforms. See ?styler:::catch_style_file_output. Testing for the cat() outputs of the public API was refactored also in this PR.

The new utility rds_to_version() can be used to convert rds objects to the old serialization format to ensure CI passing. They are introduced in this PR because this is a use case where we need to udpate current reference values. Closes #524.

[codecov-io]

Codecov Report

Merging #522 into master will decrease coverage by 0.12%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master     #522      +/-   ##
==========================================
- Coverage   90.18%   90.06%   -0.13%     
==========================================
  Files          39       40       +1     
  Lines        1732     1741       +9     
==========================================
+ Hits         1562     1568       +6     
- Misses        170      173       +3

Impacted Files	Coverage Δ
R/testing.R	68.31% <0%> (-1.39%)	⬇️
R/transform-files.R	100% <100%> (ø)	⬆️
R/testing-public-api.R	100% <100%> (ø)
R/ui.R	100% <100%> (ø)	⬆️
R/utils.R	83.33% <0%> (-5.56%)	⬇️

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