Without this change, any failed assertion in the second (or a later) test
inside a test suite has a chance of double deleting memory, resulting in
a heap corruption. See #1096 for details.
This leaves alone the test cases where we "just" use cl_git_sandbox_init()
and cl_git_sandbox_cleanup(). These methods already take good care to not
double delete a repository.
Fixes#1096
This updates all the `foreach()` type functions across the library
that take callbacks from the user to have a consistent behavior.
The rules are:
* A callback terminates the loop by returning any non-zero value
* Once the callback returns non-zero, it will not be called again
(i.e. the loop stops all iteration regardless of state)
* If the callback returns non-zero, the parent fn returns GIT_EUSER
* Although the parent returns GIT_EUSER, no error will be set in
the library and `giterr_last()` will return NULL if called.
This commit makes those changes across the library and adds tests
for most of the iteration APIs to make sure that they follow the
above rules.
