Formatting with `format_strings=true` can result in warning from rustc

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.

Reproduce the warning using the rustfmt.toml configuration and multiline string in foo.rs, then run rustc before and after rustfmt. Trace the formatting path for format_strings=true and escaped newlines. Done means rustfmt preserves valid formatting without causing the reported rustc warning.