git-gui: Verify the user has GIT_COMMITTER_IDENT before comitting.

git-gui: Verify the user has GIT_COMMITTER_IDENT before comitting.

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>