gerrit: Change repeat from 8-40 to 7-40 for hash in commentlink

The default abbreviation length in git-cli is 7 for commit
hashes, so it makes sense to link those as well instead of
requiring a minimum length of 8.

Sample:
* 49e06c8 (HEAD, gerrit/production) Dropping all search and preview url's
* 7c0f16b Fix up the mediawiki extension class a bit.
* 7c97b42 Fixing .pyconf module name
* 040cbcf Removing debug statement
* 8c6ab0f I hate when people change parameter names
* 0972bf4 Looking up udp2log socket stats by port instead of socket inode.

Change-Id: Ic2bdb1abba13ac907d889088fa08929068bfda0f
Merge "gerrit: Change repeat from 8-40 to 7-40 for hash in commentlink" into production