Merge git repository in subdirectory

Question

I'd like to merge a remote git repository in my working git repository as a subdirectory of it. I'd like the resulting repository to contain the merged history of the two repositories and also that each file of the merged-in repository retain its history as it was in the remote repository. I tried using the subtree strategy as mentioned in How to use the subtree merge strategy, but after following that procedure, although the resulting repository contains indeed the merged history of the two repositories, individual files coming from the remote one haven't retained their history (`git log' on any of them just shows a message "Merged branch...").

Also I don't want to use submodules because I do not want the two combined git repositories to be separate anymore.

Is it possible to merge a remote git repository in another one as a subdirectory with individual files coming from the remote repository retaining their history?

Thanks very much for any help.

EDIT: I'm currently trying out a solution that uses git filter-branch to rewrite the merged-in repository history. It does seem to work, but I need to test it some more. I'll return to report on my findings.

EDIT 2: In hope I make myself more clear I give the exact commands I used with git's subtree strategy, which result in apparent loss of history of the files of the remote repository. Let A be the git repo I'm currently working in and B the git repo I'd like to incorporate into A as a subdirectory of it. It did the following:

git remote add -f B <url-of-B>
git merge -s ours --no-commit B/master
git read-tree --prefix=subdir/Iwant/to/put/B/in/ -u B/master
git commit -m "Merge B as subdirectory in subdir/Iwant/to/put/B/in."

After these commands and going into directory subdir/Iwant/to/put/B/in, I see all files of B, but git log on any one of them shows just the commit message "Merge B as subdirectory in subdir/Iwant/to/put/B/in." Their file history as it is in B is lost.

What seems to work (since I'm a beginner on git I may be wrong) is the following:

git remote add -f B <url-of-B>
git checkout -b B_branch B/master  # make a local branch following B's master
git filter-branch --index-filter  
   'git ls-files -s | sed "s-"*-&subdir/Iwant/to/put/B/in/-" |
        GIT_INDEX_FILE=$GIT_INDEX_FILE.new 
                git update-index --index-info &&
        mv "$GIT_INDEX_FILE.new" "$GIT_INDEX_FILE"' HEAD 
git checkout master
git merge B_branch

The command above for filter-branch is taken from git help filter-branch, in which I only changed the subdir path.

Answer 1

git-subtree is a script designed for exactly this use case of merging multiple repositories into one while preserving history (and/or splitting history of subtrees, though that is seems to be irrelevant to this question). It is distributed as part of the git tree since release 1.7.11.

To merge a repository <repo> at revision <rev> as subdirectory <prefix>, use git subtree add as follows:

git subtree add -P <prefix> <repo> <rev>

git-subtree implements the subtree merge strategy in a more user friendly manner.

The downside is that in the merged history the files are unprefixed (not in a subdirectory). Say you merge repository a into b. As a result git log a/f1 will show you all the changes (if any) except those in the merged history. You can do:

git log --follow -- f1

but that won't show the changes other then in the merged history.

In other words, if you don't change a's files in repository b, then you need to specify --follow and an unprefixed path. If you change them in both repositories, then you have 2 commands, none of which shows all the changes.

More on it here.