git-gui: Refactor branch switch to support detached head

This is a major rewrite of the way we perform switching between
branches and the subsequent update of the working directory.  Like
core Git we now use a single code path to perform all changes: our
new checkout_op class.  We also use it for branch creation/update
as it integrates the tracking branch fetch process along with a
very basic merge (fast-forward and reset only currently).

Because some users have literally hundreds of local branches we
use the standard revision picker (with its branch filtering tool)
to select the local branch, rather than keeping all of the local
branches in the Branch menu.  The branch menu listing out all of
the available branches is simply not sane for those types of huge
repositories.

Users can now checkout a detached head by ticking off the option
in the checkout dialog.  This option is off by default for the
obvious reason, but it can be easily enabled for any local branch
by simply checking it.  We also detach the head if any non local
branch was selected, or if a revision expression was entered.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

git-gui.sh

@@ -284,7 +284,9 @@ proc git {args} {
 	return [eval exec git $args]
 }
 
-proc current-branch {} {
+proc load_current_branch {} {
+	global current_branch is_detached
+
 	set fd [open [gitdir HEAD] r]
 	if {[gets $fd ref] < 1} {
 		set ref {}
@@ -297,11 +299,13 @@ proc current-branch {} {
 		# We're on a branch.  It might not exist.  But
 		# HEAD looks good enough to be a branch.
 		#
-		return [string range $ref $len end]
+		set current_branch [string range $ref $len end]
+		set is_detached 0
 	} else {
 		# Assume this is a detached head.
 		#
-		return HEAD
+		set current_branch HEAD
+		set is_detached 1
 	}
 }
 
@@ -442,6 +446,7 @@ set MERGE_HEAD [list]
 set commit_type {}
 set empty_tree {}
 set current_branch {}
+set is_detached 0
 set current_diff_path {}
 set selected_commit_type new
 
@@ -491,7 +496,7 @@ proc repository_state {ctvar hdvar mhvar} {
 
 	set mh [list]
 
-	set current_branch [current-branch]
+	load_current_branch
 	if {[catch {set hd [git rev-parse --verify HEAD]}]} {
 		set hd {}
 		set ct initial
@@ -557,11 +562,6 @@ proc rescan {after {honor_trustmtime 1}} {
 		$ui_comm edit modified false
 	}
 
-	if {[is_enabled branch]} {
-		load_all_heads
-		populate_branch_menu
-	}
-
 	if {$honor_trustmtime && $repo_config(gui.trustmtime) eq {true}} {
 		rescan_stage2 {} $after
 	} else {
@@ -1519,6 +1519,12 @@ if {[is_enabled branch]} {
 	lappend disable_on_lock [list .mbar.branch entryconf \
 		[.mbar.branch index last] -state]
 
+	.mbar.branch add command -label {Checkout...} \
+		-command branch_checkout::dialog \
+		-accelerator $M1T-O
+	lappend disable_on_lock [list .mbar.branch entryconf \
+		[.mbar.branch index last] -state]
+
 	.mbar.branch add command -label {Rename...} \
 		-command branch_rename::dialog
 	lappend disable_on_lock [list .mbar.branch entryconf \
@@ -1739,7 +1745,7 @@ switch -- $subcommand {
 browser {
 	set subcommand_args {rev?}
 	switch [llength $argv] {
-	0 { set current_branch [current-branch] }
+	0 { load_current_branch }
 	1 { set current_branch [lindex $argv 0] }
 	default usage
 	}
@@ -1773,7 +1779,7 @@ blame {
 	unset is_path
 
 	if {$head eq {}} {
-		set current_branch [current-branch]
+		load_current_branch
 	} else {
 		set current_branch $head
 	}
@@ -2240,6 +2246,8 @@ bind $ui_diff <Button-1>   {focus %W}
 if {[is_enabled branch]} {
 	bind . <$M1B-Key-n> branch_create::dialog
 	bind . <$M1B-Key-N> branch_create::dialog
+	bind . <$M1B-Key-o> branch_checkout::dialog
+	bind . <$M1B-Key-O> branch_checkout::dialog
 }
 if {[is_enabled transport]} {
 	bind . <$M1B-Key-p> do_push_anywhere
@@ -2326,9 +2334,7 @@ user.email settings into your personal
 #
 if {[is_enabled transport]} {
 	load_all_remotes
-	load_all_heads
 
-	populate_branch_menu
 	populate_fetch_menu
 	populate_push_menu
 }