git gui: show diffs with a minimum of 1 context line

Staging hunks without context does not work, because line number
information would have to be recomputed for individual hunks.

Since it is already possible to stage individual lines using
'Stage Line for Commit', zero context diffs are not really
necessary for git gui.

Signed-off-by: Clemens Buchacher <drizzd@aon.at>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

git-gui.sh

@@ -1932,7 +1932,7 @@ proc show_more_context {} {
 
 proc show_less_context {} {
 	global repo_config
-	if {$repo_config(gui.diffcontext) >= 1} {
+	if {$repo_config(gui.diffcontext) > 1} {
 		incr repo_config(gui.diffcontext) -1
 		reshow_diff
 	}

lib/diff.tcl

@@ -175,7 +175,7 @@ proc show_diff {path w {lno {}} {scroll_pos {}}} {
 
 	lappend cmd -p
 	lappend cmd --no-color
-	if {$repo_config(gui.diffcontext) >= 0} {
+	if {$repo_config(gui.diffcontext) >= 1} {
 		lappend cmd "-U$repo_config(gui.diffcontext)"
 	}
 	if {$w eq $ui_index} {

lib/option.tcl

@@ -125,7 +125,7 @@ proc do_options {} {
 		{b gui.matchtrackingbranch {mc "Match Tracking Branches"}}
 		{b gui.fastcopyblame {mc "Blame Copy Only On Changed Files"}}
 		{i-20..200 gui.copyblamethreshold {mc "Minimum Letters To Blame Copy On"}}
-		{i-0..99 gui.diffcontext {mc "Number of Diff Context Lines"}}
+		{i-1..99 gui.diffcontext {mc "Number of Diff Context Lines"}}
 		{i-0..99 gui.commitmsgwidth {mc "Commit Message Text Width"}}
 		{t gui.newbranchtemplate {mc "New Branch Name Template"}}
 		} {