To search the subversion log file, you can pipe the output of svn log through grep.
To search for [string] in the subversion log:
svn log | grep -i -C5 [string]
The -i flag makes the search case insensitive and the -C5 includes 5 lines of output per result.
You can create a simple bash alias for this, by adding the following to /etc/bash/bashrc or ~/.bashrc
alias svngrep='svn log | grep -i -C5 '
Also see Bash aliases