rebase -i: add exec command to launch a shell command

The typical usage pattern would be to run a test (or simply a compilation
command) at given points in history.

The shell command is ran (from the worktree root), and the rebase is
stopped when the command fails, to give the user an opportunity to fix
the problem before continuing with "git rebase --continue".

This needs a little rework of skip_unnecessary_picks, which wasn't robust
enough to deal with lines like

  exec >"file    name with many spaces"

in the todolist. The new version extracts command, sha1 and rest from
each line, but outputs the line itself verbatim to avoid changing the
whitespace layout.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/lib-rebase.sh

@@ -47,6 +47,8 @@ for line in $FAKE_LINES; do
 	case $line in
 	squash|fixup|edit|reword)
 		action="$line";;
+	exec*)
+		echo "$line" | sed 's/_/ /g' >> "$1";;
 	"#")
 		echo '# comment' >> "$1";;
 	">")