git-gui: Replace null_sha1 with nullid

Both $nullid and $null_sha1 point to the same content.
Use only $nullid consistently.

This is a preliminary cleanup for adding the support of SHA256 repo.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>

Author: tiwai

git-gui.sh

@@ -1845,10 +1845,9 @@ proc short_path {path} {
 }
 
 set next_icon_id 0
-set null_sha1 [string repeat 0 40]
 
 proc merge_state {path new_state {head_info {}} {index_info {}}} {
-	global file_states next_icon_id null_sha1
+	global file_states next_icon_id nullid
 
 	set s0 [string index $new_state 0]
 	set s1 [string index $new_state 1]
@@ -1870,7 +1869,7 @@ proc merge_state {path new_state {head_info {}} {index_info {}}} {
 	elseif {$s1 eq {_}} {set s1 _}
 
 	if {$s0 eq {A} && $s1 eq {_} && $head_info eq {}} {
-		set head_info [list 0 $null_sha1]
+		set head_info [list 0 $nullid]
 	} elseif {$s0 ne {_} && [string index $state 0] eq {_}
 		&& $head_info eq {}} {
 		set head_info $index_info

lib/checkout_op.tcl

@@ -151,7 +151,7 @@ method _finish_fetch {ok} {
 }
 
 method _update_ref {} {
-	global null_sha1 current_branch repo_config
+	global nullid current_branch repo_config
 
 	set ref $new_ref
 	set new $new_hash
@@ -177,7 +177,7 @@ method _update_ref {} {
 		}
 
 		set reflog_msg "branch: Created from $new_expr"
-		set cur $null_sha1
+		set cur $nullid
 
 		if {($repo_config(branch.autosetupmerge) eq {true}
 			|| $repo_config(branch.autosetupmerge) eq {always})