It is part of the git best practices:
http://git.kernel.org/?p=git/git.git;a=blob;f=Documentation/...
http://git.kernel.org/?p=git/git.git;a=blob;f=Documentation/...
"describe changes in imperative mood, e.g. "make xyzzy do frotz" instead of "[This patch] makes xyzzy do frotz" or "[I] changed xyzzy to do frotz", as if you are giving orders to the codebase to change its behaviour."
http://git.kernel.org/?p=git/git.git;a=blob;f=Documentation/...