Dpkg::Changelog::Parse: Stop using tail(1) to read the end of the file

Instead of relying on the tail command, simply read the end of the
file ourselves, assuming a packed set of 80 character lines, reading
4096 bytes before the end, implies around 51 lines, which is close
to the 40 lines currently used.

This should be both faster and should improve portability, because even
if we are using the POSIX -n option, some systems do not have a POSIX
compliant tail(1) on the default path, such as Solaris.

Analysis-by: James Clarke <jrtc27@debian.org>

1 files changed, 4 insertions, 0 deletions

diff --git a/debian/changelog b/debian/changelog
index 8eb639c44..36b5594d8 100644
--- a/debian/changelog
+++ b/debian/changelog
@@ -99,6 +99,10 @@ dpkg (1.19.1) UNRELEASED; urgency=medium
       alias. Emit a specific perl warning until 1.20.x so that users can check
       whether the semantic change has any impact on the code, which can then
       be quiesced. Closes: #895004
+    - Dpkg::Changelog::Parse: When detecting the changelog format, read the
+      last 4KiB of the file instead of using «tail -n40», which should be
+      both faster and more portable, as the default tail(1) is not POSIX
+      compliant on all systems (c.f. Solaris).
   * Documentation:
     - Update gettext minimal version in README.
     - Add a missing dot on the dpkg-buildflags(1) «lfs» feature paragraph.