git: Show bug description if bug id passed to gb

The alias `gb` will list all bugs (or allows to query for them) by
default. But if only a single argument is provided, and that argument is
a bug id then it shows the bug.

This allows a nice workflow like:

```sh
$ gb
# -> returns list of bugs, pick one from it
$ gb <bug-id>
# -> returns detailed description of bug
```

vcs/git/config/sh/alias.d/git-bug.sh

@@ -9,7 +9,13 @@ fi
 git-bug completion zsh | . /dev/fd/0
 
 alias gbt='git-bug termui'
-alias gb="git-bug bug"
+gb() {
+    if [ "$#" -eq 1 ] && git-bug bug status "$1" >/dev/null 2>/dev/null; then
+        git-bug bug show "$1"
+    else
+        git-bug bug "$@"
+    fi
+}
 alias gbw="git-bug bug show"
 alias gbs="git-bug bug select"