Merge branch 'sc/difftool-p4merge'

* sc/difftool-p4merge:
mergetool--lib: add p4merge as a pre-configured mergetool option

diff --combined contrib/completion/git-completion.bash
index a0917d18095f63abd1935be33e8fbcdfb6b2b6fa,2a9a88968f40ec82ad980b3408a0fc389be02c32..bd66639d481db97511ef6fe3abf1b0a5d3da6ea3
--- 1/contrib/completion/git-completion.bash
--- 2/contrib/completion/git-completion.bash
+++ b/contrib/completion/git-completion.bash
@@@ -953,7 -953,7 +953,7 @@@ _git_diff (
  }
  
  __git_mergetools_common="diffuse ecmerge emerge kdiff3 meld opendiff
-                       tkdiff vimdiff gvimdiff xxdiff araxis
+                       tkdiff vimdiff gvimdiff xxdiff araxis p4merge
  "
  
  _git_difftool ()
@@@ -1224,7 -1224,7 +1224,7 @@@ _git_log (
  
  __git_merge_options="
        --no-commit --no-stat --log --no-log --squash --strategy
 -      --commit --stat --no-squash --ff --no-ff
 +      --commit --stat --no-squash --ff --no-ff --ff-only
  "
  
  _git_merge ()