Keep Porcelainish from failing by broken ident after making changes.
"empty ident not allowed" error makes commit-tree fail, so we are already safer in that we would not end up with commit objects that have bogus names on the author or committer fields. However, before commit-tree is called there are already changes made to the index file and the working tree. The operation can be resumed after fixing the environment problem, but when this triggers to a newcomer with unusable gecos, the first question becomes "what did I lose and how would I recover".
This patch modifies some Porcelainish commands to verify GIT_COMMITTER_IDENT as soon as we know we are going to make some commits before doing much damage to prevent confusion.