gitk: Make web links clickable

This makes gitk look for http or https URLs in the commit description
and make the URLs clickable.  Clicking on them will invoke an external
web browser with the URL.

The web browser command is by default "xdg-open" on Linux, "open" on
MacOS, and "cmd /c start" on Windows.  The command can be changed in
the preferences window, and it can include parameters as well as the
command name.  If it is set to the empty string then URLs will no
longer be made clickable.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>

gitk

@@ -7016,6 +7016,7 @@ proc commit_descriptor {p} {
 
 # append some text to the ctext widget, and make any SHA1 ID
 # that we know about be a clickable link.
+# Also look for URLs of the form "http[s]://..." and make them web links.
 proc appendwithlinks {text tags} {
     global ctext linknum curview
 
@@ -7032,6 +7033,18 @@ proc appendwithlinks {text tags} {
 	setlink $linkid link$linknum
 	incr linknum
     }
+    set wlinks [regexp -indices -all -inline -line \
+		    {https?://[^[:space:]]+} $text]
+    foreach l $wlinks {
+	set s2 [lindex $l 0]
+	set e2 [lindex $l 1]
+	set url [string range $text $s2 $e2]
+	incr e2
+	$ctext tag delete link$linknum
+	$ctext tag add link$linknum "$start + $s2 c" "$start + $e2 c"
+	setwlink $url link$linknum
+	incr linknum
+    }
 }
 
 proc setlink {id lk} {
@@ -7064,6 +7077,18 @@ proc setlink {id lk} {
     }
 }
 
+proc setwlink {url lk} {
+    global ctext
+    global linkfgcolor
+    global web_browser
+
+    if {$web_browser eq {}} return
+    $ctext tag conf $lk -foreground $linkfgcolor -underline 1
+    $ctext tag bind $lk <1> [list browseweb $url]
+    $ctext tag bind $lk <Enter> {linkcursor %W 1}
+    $ctext tag bind $lk <Leave> {linkcursor %W -1}
+}
+
 proc appendshortlink {id {pre {}} {post {}}} {
     global ctext linknum
 
@@ -7098,6 +7123,16 @@ proc linkcursor {w inc} {
     }
 }
 
+proc browseweb {url} {
+    global web_browser
+
+    if {$web_browser eq {}} return
+    # Use eval here in case $web_browser is a command plus some arguments
+    if {[catch {eval exec $web_browser [list $url] &} err]} {
+	error_popup "[mc "Error starting web browser:"] $err"
+    }
+}
+
 proc viewnextline {dir} {
     global canv linespc
 
@@ -11488,7 +11523,7 @@ proc create_prefs_page {w} {
 proc prefspage_general {notebook} {
     global NS maxwidth maxgraphpct showneartags showlocalchanges
     global tabstop limitdiffs autoselect autosellen extdifftool perfile_attrs
-    global hideremotes want_ttk have_ttk maxrefs
+    global hideremotes want_ttk have_ttk maxrefs web_browser
 
     set page [create_prefs_page $notebook.general]
 
@@ -11539,6 +11574,13 @@ proc prefspage_general {notebook} {
     pack configure $page.extdifff.l -padx 10
     grid x $page.extdifff $page.extdifft -sticky ew
 
+    ${NS}::entry $page.webbrowser -textvariable web_browser
+    ${NS}::frame $page.webbrowserf
+    ${NS}::label $page.webbrowserf.l -text [mc "Web browser" ]
+    pack $page.webbrowserf.l -side left
+    pack configure $page.webbrowserf.l -padx 10
+    grid x $page.webbrowserf $page.webbrowser -sticky ew
+
     ${NS}::label $page.lgen -text [mc "General options"]
     grid $page.lgen - -sticky w -pady 10
     ${NS}::checkbutton $page.want_ttk -variable want_ttk \
@@ -12310,6 +12352,7 @@ if {[tk windowingsystem] eq "win32"} {
     set bgcolor SystemWindow
     set fgcolor SystemWindowText
     set selectbgcolor SystemHighlight
+    set web_browser "cmd /c start"
 } else {
     set uicolor grey85
     set uifgcolor black
@@ -12317,6 +12360,11 @@ if {[tk windowingsystem] eq "win32"} {
     set bgcolor white
     set fgcolor black
     set selectbgcolor gray85
+    if {[tk windowingsystem] eq "aqua"} {
+	set web_browser "open"
+    } else {
+	set web_browser "xdg-open"
+    }
 }
 set diffcolors {red "#00a000" blue}
 set diffcontext 3
@@ -12390,6 +12438,7 @@ set config_variables {
     filesepbgcolor filesepfgcolor linehoverbgcolor linehoverfgcolor
     linehoveroutlinecolor mainheadcirclecolor workingfilescirclecolor
     indexcirclecolor circlecolors linkfgcolor circleoutlinecolor
+    web_browser
 }
 foreach var $config_variables {
     config_init_trace $var