Respect git "core.commentchar" setting

The comment char is read from git config if it exists, but defaults to
"#" as before.

Fixes #530
Revert "Allow help pager to clear screen on exit"

This reverts commit 5a94221027e8c83a08ee15ff03f21ab74e288f1d.

Removing "X" from `less` configuration did help to clear the screen on
exit, but also led to paginated output being swallowed up when the
output was less than one page (i.e. one line). I have no idea what
actually happens, but it's important to bring back full output.

I couldn't find a way to have paginated output cleared on exit at the
same time as having single-page output normally.

Fixes #520, reopens #368
Don't re-wrap default pull request message taken from commit

Re-wrapping to 78 chars was dangerous since it could lead to moving
a #-reference to the beginning of a line which would then get
interpreted as a comment.

Wrapping pull request message is not useful anyway since it's Markdown.
For best results when rendered, users should unwrap their text.

Fixes #548, closes #570
Merge branch 'commentchar'