* guix/scripts/lint.scm (start-with-capital-letter?): Handle empty
strings.
(check-description-style): New check for end-of-sentence space.
* tests/lint.scm: Test it.
* guix/scripts/lint.scm (check-start-article): use "string-ci=?" instead of
"string=?".
* tests/lint.scm ("synopsis: starts with 'a'",
"synopsis: starts with 'an'"): New tests.
* guix/scripts/lint.scm (check-description-style,
check-synopsis-start-upper-case): New methods.
* tests/lint.scm ("description: does not start with an upper-case letter",
"synopsis: does not start with an upper-case letter"): New tests.
This allows editors to parse warnings correctly.
* guix/scripts/lint.scm (emit-warning): Use 'format' instead of
'warning', to avoid the "guix lint: " prefix in messages.
* tests/lint.scm (call-with-warnings): Indent.