git-gui: Allow as few as 0 lines of diff context

Johannes Sixt pointed out that dropping to 0 lines of context
does allow the user to get more fine-grained hunk selection,
especially since we don't currently support "highlight and
apply (or revert)".

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

git-gui.sh

@@ -2038,17 +2038,17 @@ lappend diff_actions [list $ctxm entryconf [$ctxm index last] -state]
 $ctxm add separator
 $ctxm add command \
 	-label {Show Less Context} \
-	-command {if {$repo_config(gui.diffcontext) >= 2} {
+	-command {if {$repo_config(gui.diffcontext) >= 1} {
 		incr repo_config(gui.diffcontext) -1
 		reshow_diff
 	}}
 lappend diff_actions [list $ctxm entryconf [$ctxm index last] -state]
 $ctxm add command \
 	-label {Show More Context} \
-	-command {
+	-command {if {$repo_config(gui.diffcontext) < 99} {
 		incr repo_config(gui.diffcontext)
 		reshow_diff
-	}
+	}}
 lappend diff_actions [list $ctxm entryconf [$ctxm index last] -state]
 $ctxm add separator
 $ctxm add command -label {Options...} \

lib/diff.tcl

@@ -145,7 +145,7 @@ proc show_diff {path w {lno {}}} {
 
 	lappend cmd -p
 	lappend cmd --no-color
-	if {$repo_config(gui.diffcontext) > 0} {
+	if {$repo_config(gui.diffcontext) >= 0} {
 		lappend cmd "-U$repo_config(gui.diffcontext)"
 	}
 	if {$w eq $ui_index} {

lib/option.tcl

@@ -173,7 +173,7 @@ proc do_options {} {
 		{i-1..5 merge.verbosity {Merge Verbosity}}
 
 		{b gui.trustmtime  {Trust File Modification Timestamps}}
-		{i-1..99 gui.diffcontext {Number of Diff Context Lines}}
+		{i-0..99 gui.diffcontext {Number of Diff Context Lines}}
 		{t gui.newbranchtemplate {New Branch Name Template}}
 		} {
 		set type [lindex $option 0]