gitk: Make "git describe" output clickable, too

Automake's contribution guidelines suggest using "git describe" output
in commit logs to reference previous commits.  By contrast, in
coreutils, I had acquired the habit of using a bare SHA1 prefix (8 hex
digits), since gitk creates clickable links for that, and not for "git
describe" output.

I prefer the readability of the full "git describe" output, yet want
to retain the gitk links, so this renders as clickable not just
SHA1-like strings, but also an SHA1-like string that is prefixed by
"-g".

Signed-off-by: Jim Meyering <meyering@redhat.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>

gitk

@@ -6734,7 +6734,7 @@ proc appendwithlinks {text tags} {
 
     set start [$ctext index "end - 1c"]
     $ctext insert end $text $tags
-    set links [regexp -indices -all -inline {\m[0-9a-f]{6,40}\M} $text]
+    set links [regexp -indices -all -inline {(?:\m|-g)[0-9a-f]{6,40}\M} $text]
     foreach l $links {
 	set s [lindex $l 0]
 	set e [lindex $l 1]
@@ -6750,6 +6750,10 @@ proc appendwithlinks {text tags} {
 proc setlink {id lk} {
     global curview ctext pendinglinks
 
+    if {[string range $id 0 1] eq "-g"} {
+      set id [string range $id 2 end]
+    }
+
     set known 0
     if {[string length $id] < 40} {
 	set matches [longid $id]