core: pass all kwargs along from run_command() to start_command()

Simplify run_command() by making it nothing more than a wrapper around
start_command.

Signed-off-by: David Aguilar <davvid@gmail.com>
interaction: use core.stdout() instead of print()

Signed-off-by: David Aguilar <davvid@gmail.com>
git: simplify execute() by using core.run_command()

We can use start_command() and run_command() here now that they have
been refactored into `cola.core`.  Their old home (`cola.utils`) would
have introduced a circular dependency, which is why this had its own
eintr-retry loop.  We can eliminate all of that now, which makes this
nice and simple.

Signed-off-by: David Aguilar <davvid@gmail.com>
grep: improve tooltips and regexp mode text

Signed-off-by: David Aguilar <davvid@gmail.com>
grep: avoid searching for empty strings

Rename 'txt' to 'query' for consistency.  Avoid 'query' being empty by
checking its value in search(), which allows us to do without the extra
input_txt_changed() function.

Previously, switching modes would have triggered a search for the "hint"
text because we were using input_txt.as_unicode() instead of
input_txt.value(), which was a bug.  Fixing this causes the widget to
search for the empty string, which is avoided by checking for len(query)
in search() itself.

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