A 'dev' mode to preserve local changes in git/hg repositories

**Describe the problem/challenge you have**

We use vendir in meta-projects, and work directly in the git/hg synced sub-repositories.
After doing some changes in them, we cannot do a "vendir sync" without loosing local changes (and lazy doesn't work if we pull some ref changes).
Also, after commit & pushing changes in the sub-repositories, we would like to automatically update the vendir configuration file with the current commit id/tag of each repo.

**Describe the solution you'd like**

We think vendir could benefit from a "dev" mode, which would be enabled by using a "dev-sync" command or a "--dev" switch on the sync command.

When turned on, vendir would change its sync behavior for mercurial and git (at least) in the following way:
- if the target directory already exists, keep it
- when using already existing target :
* check if non-committed changes exists, in which case the sync is aborted
* fetch & checkout directly in the target directory

A new command "dev-baseline" (for example) would, for each vcs directory, determine what the current ref is:
- if current revision has a tag, the tag
- otherwise, the sha of the current revision
If the ref in the vendir file is not a branch (or a topic the mercurial case), and does not match the current ref determined above, the configuration is updated with the new current ref.

**Anything else you would like to add:**

The overall experience is inspired by the "hg-confman" mercurial extension that we intend to replace with vendir.

We are willing to work on an implementation for these features, and would like your opinion before diving into coding.