compat_get_comm: the two MSYS branches word-split the comm path through xargs basename

[fujibee]

compat_get_comm's two MSYS branches pipe a path into xargs basename, so any executable path containing a space is split before basename sees it.

xargs word-splits on whitespace and hands every word to a single basename call. What comes back depends on how many words the path splits into, and neither shape is the binary name:

$ echo "/usr/local/bin/node" | xargs basename          # control, no spaces
node

$ echo "/c/Program Files/nodejs/node.exe" | xargs basename    # splits into 2
Program

$ echo "/c/Program Files/My App/agent.exe" | xargs basename    # splits into 3
Program
My
agent.exe

$ basename -- "/c/Program Files/My App/agent.exe"
agent.exe

Two words lands in basename string suffix form and returns one wrong name; three or more is treated as several paths and returns several lines. Measured on macOS basename; the coreutils build shipped with Git Bash should be confirmed to behave the same before relying on the exact shape.

Two call sites on main (scripts/lib/compat.sh):

• line 136 — the /proc/$pid/cmdline path: tr '\0' '\n' < "/proc/$pid/cmdline" | head -1 | xargs basename
• line 149 — the ps -l fallback: ps -l -p "$pid" | awk 'NR==2{print $NF}' | xargs basename

Both are on the MSYS branch, where a space in the path is the common case rather than the exception (C:\Program Files\..., Application Support). Callers compare the result against a known binary name, so a wrong or multi-line return reads as "not this agent".

#770 reported the same defect on the POSIX branch (line 153) and scoped itself there deliberately; #771 fixes that one. This issue covers the two that remain.

Suggested fix, matching #771: capture the value into a variable and take basename -- "$value" of the whole string.

Not measured here: whether either branch is reached in practice on a current Git Bash install, and whether any open Windows pid-resolution report is caused by this rather than merely resembling it. Both need a Windows machine.