add analysis-bench to benchmark incremental analysis

Can be used like this:

```
$ cargo run --release -p ra_cli -- \
  analysis-bench ../chalk/ \
  --complete ../chalk/chalk-engine/src/logic.rs:94:0

loading: 225.970093ms

from scratch:   8.492373325s
no change:      445.265µs
trivial change: 95.631242ms
```

Or like this:

```
$ cargo run --release -p ra_cli -- \
  analysis-bench ../chalk/ \
  --highlight ../chalk/chalk-engine/src/logic.rs

loading: 209.873484ms

from scratch:   9.504916942s
no change:      7.731119ms
trivial change: 124.984039ms
```

"from scratch" includes initial analysis of the relevant bits of the
project

"no change" just asks the same question for the second time. It
measures overhead on assembling the answer outside of salsa.

"trivial change" doesn't do an actual salsa change, it just advances
the revision. This test how fast is salsa at validating things.

1 files changed, 13 insertions, 0 deletions

diff --git a/docs/dev/README.md b/docs/dev/README.md
index d34ff96c826..3dc37e86eba 100644
--- a/docs/dev/README.md
+++ b/docs/dev/README.md
@@ -147,3 +147,16 @@ RA_PROFILE=*@3>10        // dump everything, up to depth 3, if it takes more tha
 ```
 
 In particular, I have `export RA_PROFILE='*>10' in my shell profile.
+
+To measure time for from-scratch analysis, use something like this:
+
+```
+$ cargo run --release -p ra_cli -- analysis-stats ../chalk/
+```
+
+For measuring time of incremental analysis, use either of these:
+
+```
+$ cargo run --release -p ra_cli -- analysis-bench ../chalk/ --highlight ../chalk/chalk-engine/src/logic.rs
+$ cargo run --release -p ra_cli -- analysis-bench ../chalk/ --complete ../chalk/chalk-engine/src/logic.rs:94:0
+```