Allow updating Git sources by name (#12897)
Closes: #4567
## Summary
When adding a package with Git reference options (`--rev`, `--tag`,
`--branch`) that already has a Git source defined, use the existing Git
URL with the new reference instead of reporting an error.
This allows commands like `uv add requests --branch main` to work when
requests is already defined with a Git source in the project
configuration.
Previously, you would need to provide the whole Git url again for this
to work:
```bash
uv add git+https://github.com/psf/requests --branch main
```
<!-- What's the purpose of the change? What does it do, and why? -->
## Test Plan
- [x] Add unit tests for project
- [x] Add unit tests for script
- [x] Tested locally for project and script environments like below
### Testing Project
In a directory using the `uv` executable from this PR (via replacing
every `uv` with `cargo run --`) initialize a project and virtual
environment
```bash
uv init
uv venv
```
move into the environment
```bash
# on mac
source .venv/bin/activate
```
and add a dependency with a git url
```bash
uv add git+https://github.com/Textualize/rich --branch master
```
Then change the branch of the project to see that the branch can be
changed without need of the whole git url:
```bash
uv add rich --branch py310
```
### Testing Script
Create the following file, e.g. `script.py`:
```python
import time
from rich.progress import track
print("Starting")
for i in track(range(20), description="For example:"):
time.sleep(0.05)
print("Done")
```
Now using `uv` (referencing the executable of this PR) add the
dependency
```bash
uv add --script script.py 'git+https://github.com/Textualize/rich' --branch master
```
and check we can execute the script:
```bash
uv run script.py
```
To test the change update the branch
```bash
uv add --script script.py rich --branch py310
```
and check that the dependency is updated and the script is executed:
```bash
uv run script.py
```
<!-- How was it tested? -->
----
This is my first time contributing to `uv` (yay, 🤗) so let me know if
there is something obvious i am missing.
Unit tests will follow soon.
---------
Co-authored-by: Charlie Marsh <charlie.r.marsh@gmail.com>
