Search hint: change hint for commit messages to 'commit' instead of 'comment'

After checking with Tim and Brendan, the hint for "commit messages" should be changed from 'comment' to 'commit'.

All the other hints match the labels assigned to the serach type:

• File and directories -> hint is "file"
• Diffs -> hint is "diff"
• Content -> hint is "content"
• Committers -> hint is "committer"

Except for the one to change:

• Commits -> current hint is "comment" (will be "commit")

Reference: http://confluence.atlassian.com/display/FISHEYE/Searching+the+Repository#SearchingtheRepository-QuickSearch