commit: error out for missing commit message template

commit: error out for missing commit message template

When "git commit" was rewritten in C (v1.5.4-rc0~78^2~30,
2007-11-08), a subtle bug in --template was introduced.  If the
file named by a --template parameter is missing, previously git
would error out with a message:

	Commit template file does not exist.

but in the C version the --template parameter gets ignored and
the default template is used.

t7500 has two tests for this case which would have caught it, except
that with the default $EDITOR, the commit message template is left
unmodified, causing 'git commit' to error out and the test to
succeed.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>