Improve subtree merge command line.

Subtree merge doesn't always work as intended here.

For example, I tried these instructions with two repositories,
repository A, which contained a README file, and repository B, which
contained README and AUTHORS files. I used read-tree to put repository
B into a subdirectory of repository A, then ran the git merge
incantation affected by this patch.

With Git 1.9.1 from the Ubuntu 14.04 repositories, git-merge decided
that the correct subtree of A to merge B into was the root directory
of A, perhaps because the directory structure was similar enough to
satisfy some heuristic.

Replace the subtree strategy with recursive and explicit hint about
where the subtree is, which worked as intended. The output ("Squash
commit [...] as requested") was unchanged.

Signed-off-by: Marti Bolivar <mbolivarmullen@gmail.com>
Remove a deprecated feature of GitLab
Fix a broken reference

b4c27f5d removed the Translation section from README.asc
Update rebasing.asc

Tiny typo

Git calculate a checksum -> Git calculates a checksum
Merge pull request #195 from rahrah/patch-1

Update rebasing.asc
Merge pull request #153 from mbolivar/fix-subtree-merge

Improve subtree merge command line.
Add a link to contributing.md

ref https://github.com/progit/progit2/pull/194#issuecomment-64820254
Tweak the sentence

ref https://github.com/progit/progit2/pull/179#issuecomment-64820938
Merge remote-tracking branch 'upstream/master'

Conflicts:
	book/03-git-branching/sections/rebasing.asc
Fix #32
Merge pull request #179 from harupong/remove-gitlab-walls

Remove a deprecated feature of GitLab
Merge pull request #194 from harupong/readme-correction

Fix a broken reference
The
Correct minor typo in paragraph
Merge pull request #198 from amacfie/patch-1

Correct minor typo in paragraph
Merge remote-tracking branch 'upstream/master'

Conflicts:
	README.asc
	book/04-git-server/sections/gitlab.asc
	book/05-distributed-git/sections/contributing.asc