Bump commit subject length to 72 characters

There's not really a reason to keep the subject length so short,
since the reason to make it this short was for making nice renders
of a summary list of the git log. With 72 characters, this still
works out fine, so let's just raise it to that so that it's easier
to give slightly more descriptive change summaries.

Reviewed by: Matt Ahrens <mahrens@delphix.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Neal Gompa <ngompa@datto.com>
Closes #8250

.github/CONTRIBUTING.md

@@ -164,7 +164,7 @@ coding convention.
 ### Commit Message Formats
 #### New Changes
 Commit messages for new changes must meet the following guidelines:
-* In 50 characters or less, provide a summary of the change as the
+* In 72 characters or less, provide a summary of the change as the
 first line in the commit message.
 * A body which provides a description of the change. If necessary,
 please summarize important information such as why the proposed