cmds: adjust how we capture stdout/stderr when rebasing

3c7d62fb72eaac6ccb681ef1d61dd5f82b5c9845 modified how we launch
"git rebase" so that we do not capture stdout and stderr.

This is really only useful when git-xbase is launched from the shell via
"git cola rebase".  We do not want anything going to the console when
launched from the main UI..

Modify the Rebase command so that it captures its output by when
invoked from the main UI.  Do not capture output when invoked
via "git cola rebase".

Signed-off-by: David Aguilar <davvid@gmail.com>
qtutils: set the correct window modality for confirm()

Signed-off-by: David Aguilar <davvid@gmail.com>
qtutils: use Qt.* instead of QtCore.Qt.*

Signed-off-by: David Aguilar <davvid@gmail.com>
cmds: add a proper RevertUnstagedEdits command

Signed-off-by: David Aguilar <davvid@gmail.com>
main: add Ctrl+u shortcut for "Revert Unstaged Edits"

Closes #220

Suggested-by: @grandinj via github.com in #220
Signed-off-by: David Aguilar <davvid@gmail.com>
prefs: prefer `gvim -f` as the GUI editor

Signed-off-by: David Aguilar <davvid@gmail.com>
xbase: avoid whitespace at the start of each line

Mash the initial two `%s` specifiers together so that enabled
lines do not begin with a leading space.

Signed-off-by: David Aguilar <davvid@gmail.com>
widgets.status: disable 'Revert Unstaged Edits' when nothing is selected

Prevent the 'Revert Unstaged Edits...' action from being used when there
is nothing to revert.  This ensures that the hotkey does not activate
when there is nothing to do.

Signed-off-by: David Aguilar <davvid@gmail.com>