Since git-commit also checks that the user has a GIT_COMMITTER_IDENT
value before it lets the user make a commit we should do the same check
here in git-gui. We cache the result and assume that the user won't
do something which would change the status of GIT_COMMITTER_IDENT while
we are running.
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>