Add pre-conditions for git bash

[koppor]

I tried the tool on git bash. Got some errors.

$ ../tools/git-pulse 6
../tools/git-pulse: line 15: rev: command not found
../tools/git-pulse: line 15: rev: command not found
...
│   6 │    13 │      │   657 │    58 │ Jun 07 – Jul 07 2023 ending 1mo back   │
../tools/git-pulse: line 538: bc: command not found
....
 Cumulative total committers grew by 45 (from 618 to 663) in 5../tools/git-pulse: line 93: gnuplot: command not found

I think, it is difficult to make it running there.

However, for cygwin, it should be possible. Maybe, the following packages could be listed in the README.md?

• bc
• gnuplot-base