Consolidate some calls to uname(1) around shell startup

Most versions of fish don't run any external processes at startup, except
maybe fish_vcs_prompt.  This changed recently with a couple additions of uname.
This is probably fine but I guess we can reduce it down to one.

This change feels somewhat wrong. Not sure.  I guess we can remove it once
we provide $OSTYPE.

Note that this is also the reason why bindings don't use

	bind alt-backspace 'if test "$(uname)" = Darwin ...'

We don't want to expose a private interface in "bind" output.

share/functions/__fish_apropos.fish

@@ -4,12 +4,9 @@ if not type -q apropos
     exit
 end
 
-# Check for macOS Catalina or above. This is Darwin 19.x or above. See unames reported here:
-# https://en.wikipedia.org/wiki/Darwin_(operating_system)
-set -l sysver (uname -sr | string match -r "(Darwin) (\d+)")
-
-if test $status -eq 0 -a (count $sysver) -eq 3
-    and test $sysver[2] = Darwin -a $sysver[3] -ge 19
+# Check for macOS Catalina or above.
+if test (__fish_uname) = Darwin
+    and test (string match -r "(\d+)" "$(uname -r)") -ge 19
     and test -x /usr/libexec/makewhatis
 
     set -l dir

share/functions/__fish_shared_key_bindings.fish

@@ -21,7 +21,7 @@ function __fish_shared_key_bindings -d "Bindings shared between emacs and vi mod
     $legacy_bind --preset $argv -k left backward-char
 
     # Ctrl-left/right - these also work in vim.
-    if set -q XPC_FLAGS || set -q __CFBundleIdentifier || test (uname) = Darwin
+    if test (__fish_uname) = Darwin
         bind --preset $argv ctrl-right forward-token
         bind --preset $argv ctrl-left backward-token
     else
@@ -61,7 +61,7 @@ function __fish_shared_key_bindings -d "Bindings shared between emacs and vi mod
 
     set -l alt_right_aliases alt-right \e\[1\;9C # iTerm2 < 3.5.12
     set -l alt_left_aliases alt-left \e\[1\;9D # iTerm2 < 3.5.12
-    if set -q XPC_FLAGS || set -q __CFBundleIdentifier || test (uname) = Darwin
+    if test (__fish_uname) = Darwin
         for alt_right in $alt_left_aliases
             bind --preset $argv $alt_right nextd-or-forward-word
         end

share/functions/__fish_uname.fish

@@ -0,0 +1,6 @@
+function __fish_uname
+    if not set -q __fish_uname
+        set -g __fish_uname (uname)
+    end
+    echo -- $__fish_uname
+end

share/functions/fish_default_key_bindings.fish

@@ -55,7 +55,7 @@ function fish_default_key_bindings -d "emacs-like key binds"
     bind --preset $argv alt-u upcase-word
 
     bind --preset $argv alt-c capitalize-word
-    if set -q XPC_FLAGS || set -q __CFBundleIdentifier || test (uname) = Darwin
+    if test (__fish_uname) = Darwin
         bind --preset $argv alt-backspace backward-kill-word
         bind --preset $argv ctrl-backspace backward-kill-token
         bind --preset $argv alt-delete kill-word

share/functions/fish_git_prompt.fish

@@ -170,7 +170,7 @@ end
 
 # Decide if git is safe to run.
 # On Darwin, git is pre-installed as a stub, which will pop a dialog if you run it.
-if string match -q Darwin -- (uname) && string match -q /usr/bin/git -- (command -s git) && type -q xcode-select && type -q xcrun
+if string match -q Darwin -- (__fish_uname) && string match -q /usr/bin/git -- (command -s git) && type -q xcode-select && type -q xcrun
     if not xcode-select --print-path &>/dev/null
         # Only the stub git is installed.
         # Do not try to run it.