Support git config commentChar

[coady]

Comment lines in git are configurable, but appear to be hard-coded to # in stgit. New patches append

# Please enter the message for your patch. Lines starting with
# '#' will be ignored. An empty message aborts the new patch.
# The patch name and author information may also be modified.

and strip all lines starting with # regardless of the git config core.commentChar setting.

The use case is that commit text is commonly used in markdown descriptions - such as GitHub - where # is used for headings.