gitk: Add option to specify the default commit on command line

Other GUI tools may need to start gitk and make it automatically
select a certain commit.  This adds a new command-line option
--select-commit=id to make that possible.

Signed-off-by: Alexander Gavrilov <angavrilov@gmail.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>

gitk

@@ -418,10 +418,12 @@ proc stop_rev_list {view} {
 }
 
 proc reset_pending_select {selid} {
-    global pending_select mainheadid
+    global pending_select mainheadid selectheadid
 
     if {$selid ne {}} {
 	set pending_select $selid
+    } elseif {$selectheadid ne {}} {
+	set pending_select $selectheadid
     } else {
 	set pending_select $mainheadid
     }
@@ -1609,6 +1611,7 @@ proc getcommit {id} {
 proc readrefs {} {
     global tagids idtags headids idheads tagobjid
     global otherrefids idotherrefs mainhead mainheadid
+    global selecthead selectheadid
 
     foreach v {tagids idtags headids idheads otherrefids idotherrefs} {
 	catch {unset $v}
@@ -1655,6 +1658,12 @@ proc readrefs {} {
 	    set mainhead [string range $thehead 11 end]
 	}
     }
+    set selectheadid {}
+    if {$selecthead ne {}} {
+	catch {
+	    set selectheadid [exec git rev-parse --verify $selecthead]
+	}
+    }
 }
 
 # skip over fake commits
@@ -9865,6 +9874,9 @@ if {![file isdirectory $gitdir]} {
     exit 1
 }
 
+set selecthead {}
+set selectheadid {}
+
 set revtreeargs {}
 set cmdline_files {}
 set i 0
@@ -9876,6 +9888,9 @@ foreach arg $argv {
 	    set cmdline_files [lrange $argv [expr {$i + 1}] end]
 	    break
 	}
+	"--select-commit=*" {
+	    set selecthead [string range $arg 16 end]
+	}
 	"--argscmd=*" {
 	    set revtreeargscmd [string range $arg 10 end]
 	}
@@ -9886,6 +9901,10 @@ foreach arg $argv {
     incr i
 }
 
+if {$selecthead eq "HEAD"} {
+    set selecthead {}
+}
+
 if {$i >= [llength $argv] && $revtreeargs ne {}} {
     # no -- on command line, but some arguments (other than --argscmd)
     if {[catch {