Test case:

  $ cat >> $HGRCPATH << EOF
  > [diff]
  > git=1
  > [experimental]
  > evolution = createmarkers
  > EOF

  $ hg init repo
  $ cd repo

  $ for i in 1 2; do
  >   echo $i >> a
  >   hg commit -A a -m $i
  > done

  $ hg update 0 -q
  $ echo 2 >> a
  $ touch b
  $ hg commit -A b -A a -m '2 with b' -q

the following works as expected

  $ hg log -G -p -T '{rev}:{node|short} {desc}'
  @  2:281cd4d94b56 2 with bdiff --git a/a b/a
  |  --- a/a
  |  +++ b/a
  |  @@ -1,1 +1,2 @@
  |   1
  |  +2
  |  diff --git a/b b/b
  |  new file mode 100644
  |
  | o  1:1ed24be7e7a0 2diff --git a/a b/a
  |/   --- a/a
  |    +++ b/a
  |    @@ -1,1 +1,2 @@
  |     1
  |    +2
  |
  o  0:eff892de26ec 1diff --git a/a b/a
     new file mode 100644
     --- /dev/null
     +++ b/a
     @@ -0,0 +1,1 @@
     +1

with -p FILE, it produces wrong output:

  $ hg log -G -p -f a -T '{rev}:{node|short} {desc}'
  @  2:281cd4d94b56 2 with b
  |
  o  0:eff892de26ec 1diff --git a/a b/a
     new file mode 100644
     --- /dev/null
     +++ b/a
     @@ -0,0 +1,1 @@
     +1

expected output is:

  $ hg log -G -p -f a -T '{rev}:{node|short} {desc}'
  @  2:281cd4d94b56 2 with b
  |  --- a/a
  |  +++ b/a
  |  @@ -1,1 +1,2 @@
  |   1
  |  +2
  |
  o  0:eff892de26ec 1diff --git a/a b/a
     new file mode 100644
     --- /dev/null
     +++ b/a
     @@ -0,0 +1,1 @@
     +1