new: fix dead patterns in the generated debian/.gitignore

Patterns containing a slash are anchored relative to the directory
holding the .gitignore, so 'debian/files' inside debian/.gitignore
only ever matched debian/debian/files: every generated pattern was
dead and debhelper artifacts showed up as untracked. Write the
patterns relative to debian/ instead.

src/new/debian.rs

@@ -306,17 +306,19 @@ fn copyright(opts: &NewOptions) -> OutputFile {
     OutputFile::new("debian/copyright", out)
 }
 
-/// `debian/.gitignore`: the debhelper build artifacts.
+/// `debian/.gitignore`: the debhelper build artifacts. The patterns are
+/// relative to `debian/` itself (a `debian/`-prefixed pattern would be
+/// anchored to `debian/debian/` inside this file, per gitignore(5)).
 fn debian_gitignore(opts: &NewOptions) -> OutputFile {
     OutputFile::new(
         "debian/.gitignore",
         format!(
-            "debian/files\n\
-             debian/.debhelper/\n\
-             debian/*.log\n\
-             debian/{}/\n\
-             debian/debhelper-build-stamp\n\
-             debian/*.substvars\n",
+            "files\n\
+             .debhelper/\n\
+             *.log\n\
+             {}/\n\
+             debhelper-build-stamp\n\
+             *.substvars\n",
             opts.name
         ),
     )
@@ -766,8 +768,8 @@ mod tests {
         let g = super::debian_gitignore(&opts());
         assert_eq!(
             g.contents,
-            "debian/files\ndebian/.debhelper/\ndebian/*.log\ndebian/mytool/\n\
-             debian/debhelper-build-stamp\ndebian/*.substvars\n"
+            "files\n.debhelper/\n*.log\nmytool/\n\
+             debhelper-build-stamp\n*.substvars\n"
         );
     }