maint: patch gitlog-to-changelog to reduce bogus warnings.

maint: patch gitlog-to-changelog to reduce bogus warnings.

Rather than trying to find a new useful warning among the 55
expected warnings from vanilla gitlog-to-changelog, patch it to
elide the bogus warnings from duplicate log entries.
* gl/bulid-aux/gitlog-to-changelog.diff: Omit-from-ChangeLog is
a new log entry tag to skip legitimate log duplicates.
* build-aux/git-log-fix: Instead of deleting all content of
duplicates and generating a warning, append the new
Omit-from-ChangeLog tag to elide silently.
* Makefile.am *($(changelog)): Remove the 'expect 55 warnings'
echo.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>