gistfile1.tcl

#!/usr/bin/tclsh8.5
#
# Usage: unmerged branch1 branch2

proc getlog branch {
    lrange [split [exec git log $branch --oneline] "\n"] 0 100
}

proc diff {title c1 c2} {
    puts "\n$title"
    foreach commit1 $c1 {
        set found 0
        set sha [lindex [split $commit1] 0]
        set msg [join [lrange [split $commit1] 1 end]]
        foreach commit2 $c2 {
            set msg2 [join [lrange [split $commit2] 1 end]]
            if {$msg eq $msg2} {
                set found 1
                break
            }
        }
        if {!$found} {
            puts "$commit1"
        }
    }
}

if {[llength $::argv] != 2} {
    puts stderr "Usage: unmerged branch1 branch2"
    exit 1
}
set branch1 [lindex $::argv 0]
set branch2 [lindex $::argv 1]

set c1 [getlog $branch1]
set c2 [getlog $branch2]
diff "Only in $branch1" [lrange $c1 0 50] $c2
diff "Only in $branch2" [lrange $c2 0 50] $c1

I arrived here while looking to solve a slightly different problem: I'd like to see the branches (and commits therein) that aren't merged with master (or another branch that I specify). I found this 'git-unmerged' script very useful for solving that: http://mutuallyhuman.com/blog/2009/07/02/finding-unmerged-commits-with-git-unmerged

Posting here in case others find themselves in a similar situation.