MRELEASE-875

Even though I have commit rights myself, I would like to have other to take a look at this first.
This PR fixes an issue voted my a couple of people https://jira.codehaus.org/browse/MRELEASE-875
The fix will allow to do a release of artifacts/repos not having a pom.xml in the root of the git-repository.

The PR fixes the issue for `maven-scm-provider-jgit` and `maven-scm-provider-gitexe`

given the follwing directory layout:

```
.git
subdir1/
subdir1/subdir2/pom.xml
subdir1/subdir2/module1/pom.xml
```
the following will be possible:
in subdir2:
```
$ mvn -f pom.xml release:prepare release:perform 
```
in subdir1:
```
$ mvn -f subdir2/pom.xml release:prepare release:perform 
```
in root dir:
```
$ mvn -f subdir1/subdir2/pom.xml release:prepare release:perform 
```