Rework git toasts (#26420)

The notifications from git output could take up variable amounts of
screen space, and they were quite obnoxious when a git command printed
lots of output, such as fetching many new branches or verbose push
hooks.

This change makes the push/pull/fetch buttons trigger a small
notification toast, based on the output of the command that was ran. For
errors or commands with more output the user may want to see, there's an
"Open Log" button which opens a new buffer with the output of that
command.

It also uses this behavior for long error notifications for other git
commands like `commit` and `checkout`. The output of those commands can
be quite long due to arbitrary githooks running.

Release Notes:

- N/A

---------

Co-authored-by: Mikayla Maki <mikayla.c.maki@gmail.com>

crates/git_ui/Cargo.toml

@@ -36,6 +36,7 @@ linkme.workspace = true
 log.workspace = true
 menu.workspace = true
 multi_buffer.workspace = true
+notifications.workspace = true
 panel.workspace = true
 picker.workspace = true
 postage.workspace = true