gitk: Speed up the reading of references

We were doing two execs for each tag - one to map the tag ID to a
commit ID and one to read the contents of the tag for later display.
This speeds up the process by not reading the contents of the tag
(instead it is read later if needed), and by using the -d flag to
git show-ref, which gives us refs/tags/foo^{} lines which give us
the commit ID.  Also this uses string operations instead of regexps.

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

gitk

@@ -387,47 +387,39 @@ proc getcommit {id} {
 }
 
 proc readrefs {} {
-    global tagids idtags headids idheads tagcontents
+    global tagids idtags headids idheads tagobjid
     global otherrefids idotherrefs mainhead mainheadid
 
     foreach v {tagids idtags headids idheads otherrefids idotherrefs} {
 	catch {unset $v}
     }
-    set refd [open [list | git show-ref] r]
-    while {0 <= [set n [gets $refd line]]} {
-	if {![regexp {^([0-9a-f]{40}) refs/([^^]*)$} $line \
-	    match id path]} {
-	    continue
-	}
-	if {[regexp {^remotes/.*/HEAD$} $path match]} {
-	    continue
-	}
-	if {![regexp {^(tags|heads)/(.*)$} $path match type name]} {
-	    set type others
-	    set name $path
-	}
-	if {[regexp {^remotes/} $path match]} {
-	    set type heads
-	}
-	if {$type == "tags"} {
-	    set tagids($name) $id
-	    lappend idtags($id) $name
-	    set obj {}
-	    set type {}
-	    set tag {}
-	    catch {
-		set commit [exec git rev-parse "$id^0"]
-		if {$commit != $id} {
-		    set tagids($name) $commit
-		    lappend idtags($commit) $name
-		}
-	    }		
-	    catch {
-	        set tagcontents($name) [exec git cat-file tag $id]
+    set refd [open [list | git show-ref -d] r]
+    while {[gets $refd line] >= 0} {
+	if {[string index $line 40] ne " "} continue
+	set id [string range $line 0 39]
+	set ref [string range $line 41 end]
+	if {![string match "refs/*" $ref]} continue
+	set name [string range $ref 5 end]
+	if {[string match "remotes/*" $name]} {
+	    if {![string match "*/HEAD" $name]} {
+		set headids($name) $id
+		lappend idheads($id) $name
 	    }
-	} elseif { $type == "heads" } {
+	} elseif {[string match "heads/*" $name]} {
+	    set name [string range $name 6 end]
 	    set headids($name) $id
 	    lappend idheads($id) $name
+	} elseif {[string match "tags/*" $name]} {
+	    # this lets refs/tags/foo^{} overwrite refs/tags/foo,
+	    # which is what we want since the former is the commit ID
+	    set name [string range $name 5 end]
+	    if {[string match "*^{}" $name]} {
+		set name [string range $name 0 end-3]
+	    } else {
+		set tagobjid($name) $id
+	    }
+	    set tagids($name) $id
+	    lappend idtags($id) $name
 	} else {
 	    set otherrefids($name) $id
 	    lappend idotherrefs($id) $name
@@ -6777,14 +6769,19 @@ proc listrefs {id} {
 }
 
 proc showtag {tag isnew} {
-    global ctext tagcontents tagids linknum
+    global ctext tagcontents tagids linknum tagobjid
 
     if {$isnew} {
 	addtohistory [list showtag $tag 0]
     }
     $ctext conf -state normal
     clear_ctext
     set linknum 0
+    if {![info exists tagcontents($tag)]} {
+	catch {
+	    set tagcontents($tag) [exec git cat-file tag $tagobjid($tag)]
+	}
+    }
     if {[info exists tagcontents($tag)]} {
 	set text $tagcontents($tag)
     } else {