Git prompt: Only shorten sha if needed

This speeds up the common case when IO is slow, e.g. when used with
sshfs.

We only use the short sha for figuring out whether the state is
valid (for which a long sha should also work) and for display when HEAD
is detached (I think that's the correct git-ism).

Working towards #3083.

share/functions/__fish_git_prompt.fish

@@ -351,17 +351,14 @@ function __fish_git_prompt --description "Prompt function for Git"
 	if not command -s git >/dev/null
 		return 1
 	end
-	set -l repo_info (command git rev-parse --git-dir --is-inside-git-dir --is-bare-repository --is-inside-work-tree --short HEAD ^/dev/null)
+	set -l repo_info (command git rev-parse --git-dir --is-inside-git-dir --is-bare-repository --is-inside-work-tree HEAD ^/dev/null)
 	test -n "$repo_info"; or return
 
 	set -l git_dir         $repo_info[1]
 	set -l inside_gitdir   $repo_info[2]
 	set -l bare_repo       $repo_info[3]
 	set -l inside_worktree $repo_info[4]
-	set -l short_sha
-	if test (count $repo_info) = 5
-		set short_sha $repo_info[5]
-	end
+	set -q repo_info[5]; and set -l sha $repo_info[5]
 
 	set -l rbc (__fish_git_prompt_operation_branch_bare $repo_info)
 	set -l r $rbc[1] # current operation
@@ -388,7 +385,7 @@ function __fish_git_prompt --description "Prompt function for Git"
 				set -l config (command git config --bool bash.showDirtyState)
 				if test "$config" != "false"
 					set w (__fish_git_prompt_dirty)
-					set i (__fish_git_prompt_staged $short_sha)
+					set i (__fish_git_prompt_staged $sha)
 				end
 			end
 
@@ -462,11 +459,11 @@ end
 ### helper functions
 
 function __fish_git_prompt_staged --description "__fish_git_prompt helper, tells whether or not the current branch has staged files"
-	set -l short_sha $argv[1]
+	set -l sha $argv[1]
 
 	set -l staged
 
-	if test -n "$short_sha"
+	if test -n "$sha"
 		command git diff-index --cached --quiet HEAD --; or set staged $___fish_git_prompt_char_stagedstate
 	else
 		set staged $___fish_git_prompt_char_invalidstate
@@ -534,10 +531,7 @@ function __fish_git_prompt_operation_branch_bare --description "__fish_git_promp
 	set -l git_dir         $argv[1]
 	set -l inside_gitdir   $argv[2]
 	set -l bare_repo       $argv[3]
-	set -l short_sha
-	if test (count $argv) = 5
-		set short_sha $argv[5]
-	end
+	set -q argv[5]; and set -l sha $argv[5]
 
 	set -l branch
 	set -l operation
@@ -598,6 +592,7 @@ function __fish_git_prompt_operation_branch_bare --description "__fish_git_promp
 							command git describe --tags --exact-match HEAD
 						end ^/dev/null; set os $status)
 			if test $os -ne 0
+				set -q sha; and set -l short_sha (command git rev-parse --short $sha)
 				if test -n "$short_sha"
 					set branch $short_sha...
 				else