git-gui: add gui and pick as explicit subcommands

git-gui accepts subcommands blame | browser | citool, and assumes the
subcommand is 'gui' if none is actually given, But, git-gui also has a
repository picker (choose_repository::pick) that can create a new
repository + worktree, or choose an existing one, switch to that, and
the run the gui. The user has no direct control over invoking the
picker, instead the picker is triggered by failure in the repository /
worktree discovery process: this includes being started in a directory
not controlled by git, which is probably the intended use case.

The picker can appear when the user has no intention of creating a new
worktree, and the user cannot use the picker to create a new worktree
inside another.

So, add two explicit subcommands:
    gui  - Run the gui if repository/worktree discovery succeeds, or die
           with an error message, but never run the picker.
    pick - First run the picker, regardless, then start the gui in
           the chosen worktree.

Nothing in this changes the prior behavior, the alternates above must be
explicitly selected to see any change.

Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>

git-gui.sh

@@ -1024,6 +1024,8 @@ proc load_config {include_global} {
 ##
 ## feature option selection
 
+enable_option picker
+enable_option gitdir_discovery
 if {[regexp {^git-(.+)$} [file tail $argv0] _junk subcommand]} {
 	unset _junk
 } else {
@@ -1035,6 +1037,9 @@ if {$subcommand eq {gui.sh}} {
 if {$subcommand eq {gui} && [llength $argv] > 0} {
 	set subcommand [lindex $argv 0]
 	set argv [lrange $argv 1 end]
+	if {$subcommand eq {gui}} {
+		disable_option picker
+	}
 }
 
 enable_option multicommit
@@ -1050,6 +1055,7 @@ blame {
 	disable_option multicommit
 	disable_option branch
 	disable_option transport
+	disable_option picker
 }
 citool {
 	enable_option singlecommit
@@ -1058,6 +1064,7 @@ citool {
 	disable_option multicommit
 	disable_option branch
 	disable_option transport
+	disable_option picker
 
 	while {[llength $argv] > 0} {
 		set a [lindex $argv 0]
@@ -1080,6 +1087,9 @@ citool {
 		set argv [lrange $argv 1 end]
 	}
 }
+pick {
+	disable_option gitdir_discovery
+}
 }
 
 ######################################################################
@@ -1168,7 +1178,7 @@ proc unset_gitdir_vars {} {
 
 # find repository
 set _gitdir {}
-if {$_gitdir eq {}} {
+if {[is_enabled gitdir_discovery]} {
 	if {[catch {
 			set _gitdir [git rev-parse --absolute-git-dir]
 		} err]} {
@@ -1180,7 +1190,7 @@ if {$_gitdir eq {}} {
 }
 
 set picked 0
-if {$_gitdir eq {}} {
+if {$_gitdir eq {} && [is_enabled picker]} {
 	unset_gitdir_vars
 	load_config 1
 	apply_config
@@ -1195,6 +1205,12 @@ if {$_gitdir eq {}} {
 	set picked 1
 }
 
+if {$_gitdir eq {}} {
+	catch {wm withdraw .}
+	error_popup [strcat [mc "Git directory not found:"] "\n\n$err"]
+	exit 1
+}
+
 # find worktree, continue without if not required
 if {[catch {
 		set _gitworktree [git rev-parse --show-toplevel]
@@ -3103,14 +3119,15 @@ blame {
 	return
 }
 citool -
-gui {
+gui -
+pick {
 	if {[llength $argv] != 0} {
 		usage
 	}
 	# fall through to setup UI for commits
 }
 default {
-	set err "[mc usage:] $argv0 \[{blame|browser|citool}\]"
+	set err "[mc usage:] $argv0 \[{blame|browser|citool|gui|pick}\]"
 	if {[tk windowingsystem] eq "win32"} {
 		wm withdraw .
 		tk_messageBox -icon error -message $err \