Formatting with format_strings=true can result in warning from rustc

[martinvonz]

The following few lines show the issue:

# Write config and a file with a multi-line string
$ echo -e 'format_strings = true\nmax_width = 100' > rustfmt.toml
$ echo -e 'fn main() { let _about_message = "An experimental VCS

To get started, see the tutorial at https://github.com/martinvonz/jj/blob/main/docs/tutorial.md.

Note that `jj <command> --help` gives more verbose help than `jj <command> -h` or `jj help <command>`."; }' > foo.rs

# Compilation succeeds without warnings before formatting
$ rustc foo.rs

# Format the file
$ rustfmt foo.rs

# Compilation produces a warning after formatting
$ rustc foo.rs
warning: multiple lines skipped by escaped newline
 --> foo.rs:4:97
  |
4 |   To get started, see the tutorial at https://github.com/martinvonz/jj/blob/main/docs/tutorial.md.\
  |  _________________________________________________________________________________________________^
5 | |
6 | |
7 | | Note that `jj <command> --help` gives more verbose help than `jj <command> -h` or `jj help \
  | |_ skipping everything up to and including this point

warning: 1 warning emitted

The second line of the string can be shortened to well under 100 characters and even under 70 characters and it will still reproduce.