gitk: Fix bugs in blaming code

The "show origin of this line" function wasn't working when gitk was
run in a subdirectory, since it passed the path relative to the
top-level directory to git blame.  This fixes it by passing the
absolute path to the file instead of the relative path.

The same problem occurs when running git gui blame, except that
git gui blame appears not to be able to accept an absolute path to the
file, so we make a relative path using a new [make_relative] function.

Finally, this fixes a bug in [show_line_source] where we weren't
setting id, resulting in an error when trying to find the origin of
a line in the fake commit for local changes not checked in, when its
parent was a real commit (i.e. there were no changes checked in).

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

gitk

@@ -3318,8 +3318,27 @@ proc index_sha1 {fname} {
     return {}
 }
 
+# Turn an absolute path into one relative to the current directory
+proc make_relative {f} {
+    set elts [file split $f]
+    set here [file split [pwd]]
+    set ei 0
+    set hi 0
+    set res {}
+    foreach d $here {
+	if {$ei < $hi || $ei >= [llength $elts] || [lindex $elts $ei] ne $d} {
+	    lappend res ".."
+	} else {
+	    incr ei
+	}
+	incr hi
+    }
+    set elts [concat $res [lrange $elts $ei end]]
+    return [eval file join $elts]
+}
+
 proc external_blame {parent_idx {line {}}} {
-    global flist_menu_file
+    global flist_menu_file gitdir
     global nullid nullid2
     global parentlist selectedline currentid
 
@@ -3338,7 +3357,12 @@ proc external_blame {parent_idx {line {}}} {
     if {$line ne {} && $line > 1} {
 	lappend cmdline "--line=$line"
     }
-    lappend cmdline $base_commit $flist_menu_file
+    set f [file join [file dirname $gitdir] $flist_menu_file]
+    # Unfortunately it seems git gui blame doesn't like
+    # being given an absolute path...
+    set f [make_relative $f]
+    lappend cmdline $base_commit $f
+    puts "cmdline={$cmdline}"
     if {[catch {eval exec $cmdline &} err]} {
 	error_popup "[mc "git gui blame: command failed:"] $err"
     }
@@ -3382,6 +3406,8 @@ proc show_line_source {} {
 		    error_popup [mc "Error reading index: %s" $err]
 		    return
 		}
+	    } else {
+		set id $parents($curview,$currentid)
 	    }
 	} else {
 	    set id [lindex $parents($curview,$currentid) $pi]
@@ -3398,7 +3424,7 @@ proc show_line_source {} {
     } else {
 	lappend blameargs $id
     }
-    lappend blameargs -- $flist_menu_file
+    lappend blameargs -- [file join [file dirname $gitdir] $flist_menu_file]
     if {[catch {
 	set f [open $blameargs r]
     } err]} {