Use 'fast-forward' all over the place

It's a compound word.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/howto/maintain-git.txt

@@ -59,7 +59,7 @@ The policy.
    not yet pass the criteria set for 'next'.
 
  - The tips of 'master', 'maint' and 'next' branches will always
-   fast forward, to allow people to build their own
+   fast-forward, to allow people to build their own
    customization on top of them.
 
  - Usually 'master' contains all of 'maint', 'next' contains all

Documentation/howto/revert-branch-rebase.txt

@@ -85,7 +85,7 @@ Fortunately I did not have to; what I have in the current branch
 
 ------------------------------------------------
 $ git checkout master
-$ git merge revert-c99 ;# this should be a fast forward
+$ git merge revert-c99 ;# this should be a fast-forward
 Updating from 10d781b9caa4f71495c7b34963bef137216f86a8 to e3a693c...
  cache.h        |    8 ++++----
  commit.c       |    2 +-
@@ -95,7 +95,7 @@ Updating from 10d781b9caa4f71495c7b34963bef137216f86a8 to e3a693c...
  5 files changed, 8 insertions(+), 8 deletions(-)
 ------------------------------------------------
 
-There is no need to redo the test at this point.  We fast forwarded
+There is no need to redo the test at this point.  We fast-forwarded
 and we know 'master' matches 'revert-c99' exactly.  In fact:
 
 ------------------------------------------------

Documentation/howto/update-hook-example.txt

@@ -76,7 +76,7 @@ case "$1" in
     if expr "$2" : '0*$' >/dev/null; then
       info "The branch '$1' is new..."
     else
-      # updating -- make sure it is a fast forward
+      # updating -- make sure it is a fast-forward
       mb=$(git-merge-base "$2" "$3")
       case "$mb,$2" in
         "$2,$mb") info "Update is fast-forward" ;;